Makefile: fix out of tree builds of .m files

should fix fate failures on darwin

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Makefile

@@ -4,6 +4,7 @@ include config.mak
 vpath %.c    $(SRC_PATH)
 vpath %.cpp  $(SRC_PATH)
 vpath %.h    $(SRC_PATH)
+vpath %.m    $(SRC_PATH)
 vpath %.S    $(SRC_PATH)
 vpath %.asm  $(SRC_PATH)
 vpath %.rc   $(SRC_PATH)