egl/android: fix incompatible pointer in free(device_name)

egl/android: fix incompatible pointer in free(device_name)

Fixes the following building error:

external/mesa/src/egl/drivers/dri2/platform_android.c:1476:12:
error: passing 'const char *' to parameter of type 'void *' discards qualifiers [-Werror,-Wincompatible-pointer-types-discards-qualifiers]
      free(device_name);
           ^~~~~~~~~~~
bionic/libc/include/malloc.h:36:17: note: passing argument to parameter 'p' here
void free(void* p);
                ^
1 error generated.

Fixes: da5d83c ("egl/android: determine how to open device at runtime")
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>