HACKING.md: Update with C99 being our requirement
HACKING.md: Update with C99 being our requirement

We have been requiring at least parts of C99 like stdint.h for some
time now, but HACKING.md has not reflected that until now.

Closes: https://github.com/gphoto/libgphoto2/issues/724

gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2,gphoto/libgphoto2