Makefile: Switch to using libretro-common subrepo

Instead of integrating the content of the libretro-common repository
inside this core, reference it as a git subrepo, just like it is
currently done with GNU Lightning and Lightrec.

The advantage is that the latest code from the main libretro-common
repository can be fetched easily now.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>