Use function with easier input for generating PRX module files

[sharkwouter]

This would make using this a bit easier. Right now you have to do this:

add_prx_module(TARGET myprx
    SRC_FILES main.c extra.c
    EXP_FILES main.exp extra.exp
)

This change would allow you to do this instead:

add_prx_module(myprx
    main.c
    extra.c
    main.exp
    extra.exp
)

The function will figure out which of the files specified are exp files and generate c files from them without the user needing to make a distinction.

[fjtrujy]

Good improvement!