iio-sensors: avoid memset a dynamic class

iio-sensors: avoid memset a dynamic class

A dynamic class doesn't support static initialization. The compiler
complains:

hardware/libsensors/iio-sensors.cpp:119:9: warning: destination for this 'memset' call is a pointer to dynamic class 'SensorBase'; vtable pointer will be overwritten [-Wdynamic-class-memaccess]
        memset(this, 0, sizeof(SensorBase));
        ~~~~~~ ^

Move all data members of SensorBase to a trivial class which could be
statically initialized.

Refer: http://en.cppreference.com/w/cpp/types/is_trivial