Makefile: make ppc/sha1ppc.o depend on GIT-CFLAGS

Makefile: make ppc/sha1ppc.o depend on GIT-CFLAGS

The %.o: %.S pattern rule should depend on GIT-CFLAGS to avoid
trouble when ALL_CFLAGS changes.

The pattern only applies to one file (ppc/sha1ppc.S) and that
file does not use any #ifdefs, so leaving the dependency out is
probably harmless.  Nevertheless, it is safer to include the
dependency in case future code's behavior does depend on the
build flags.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>