Fix missing Metal object header dependencies

[IngeniousIdiocy]

Rebuild ds4_metal.o when ds4.h or ds4_image.h changes.

Build passes on macOS; make -n -W <header> ds4_metal.o confirms both headers now trigger recompilation.

[emilianbold]

This is correct but I wonder if we should't use https://www.gnu.org/software/make/manual/make.html#Generating-Prerequisites-Automatically or something similar so these makefile header dependencies don't drift so much.

[IngeniousIdiocy]

This is correct but I wonder if we should't use https://www.gnu.org/software/make/manual/make.html#Generating-Prerequisites-Automatically or something similar so these makefile header dependencies don't drift so much.

entirely up to you guys. I normally play in my own branch of ds4 doing things not really appropriate for the whole project but this one felt like I owed a PR. thanks for all the work in maintaining it.