msvc: fix dependencies of compat/msvc.c

The file compat/msvc.c includes compat/mingw.c, which means that we have
to recompile compat/msvc.o if compat/mingw.c changes.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

config.mak.uname

@@ -408,6 +408,8 @@ else
 	BASIC_CFLAGS += -Zi -MDd
 endif
 	X = .exe
+
+compat/msvc.o: compat/msvc.c compat/mingw.c GIT-CFLAGS
 endif
 ifeq ($(uname_S),Interix)
 	NO_INITGROUPS = YesPlease