Add .inc as a header file type.
Add .inc as a header file type.

Currently, if you were to add a directory that only contained .inc
files, this file would not be passed as an include path to the compiler.
This ensures that .inc files are also considered header files.

mbedmicro/mbed,mbedmicro/mbed,mbedmicro/mbed,kjbracey-arm/mbed,andcor02/mbed-os,mbedmicro/mbed,andcor02/mbed-os,mbedmicro/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed,kjbracey-arm/mbed,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os,andcor02/mbed-os