activity: fix build error by including <errno.h>

activity: fix build error by including <errno.h>

Fixes the following building errors:

hardware/intel/libsensors/activity.c:314:48: error: use of undeclared identifier 'errno'
                ALOGE("Error %s creating socket\n", strerror(errno));
                                                             ^
hardware/intel/libsensors/activity.c:320:47: error: use of undeclared identifier 'errno'
                ALOGE("Error %s binding socket\n", strerror(errno));
                                                            ^
hardware/intel/libsensors/activity.c:326:66: error: use of undeclared identifier 'errno'
                ALOGW("Error %s setting socket to listening state\n", strerror(errno));
                                                                               ^
hardware/intel/libsensors/activity.c:349:54: error: use of undeclared identifier 'errno'
                        ALOGE("Error %s accepting connection\n", strerror(errno));
                                                                          ^
hardware/intel/libsensors/activity.c:355:65: error: use of undeclared identifier 'errno'
                        ALOGE("Error %s in receiving message, conn = %d\n", strerror(errno), conn);
                                                                                     ^
hardware/intel/libsensors/activity.c:508:62: error: use of undeclared identifier 'errno'
                fprintf(stderr, "Error \"%s\" creating socket\n", strerror(errno));
                                                                           ^
hardware/intel/libsensors/activity.c:509:10: error: use of undeclared identifier 'errno'
                return errno;
                       ^
hardware/intel/libsensors/activity.c:514:67: error: use of undeclared identifier 'errno'
                fprintf(stderr, "Error \"%s\" connecting to server\n", strerror(errno));
                                                                                ^
hardware/intel/libsensors/activity.c:515:10: error: use of undeclared identifier 'errno'
                return errno;
                       ^
hardware/intel/libsensors/activity.c:520:72: error: use of undeclared identifier 'errno'
                fprintf(stderr, "Error \"%s\" sending message to server\n", strerror(errno));
                                                                                     ^
hardware/intel/libsensors/activity.c:521:10: error: use of undeclared identifier 'errno'
                return errno;
                       ^
hardware/intel/libsensors/activity.c:526:73: error: use of undeclared identifier 'errno'
                fprintf(stderr, "Error \"%s\" getting answer from server\n", strerror(errno));
                                                                                      ^
hardware/intel/libsensors/activity.c:527:10: error: use of undeclared identifier 'errno'
                return errno;
                       ^
13 errors generated.