Make objects in $(export-objs) only depend on themselves

Currently, if dependencies for one object file listed in $(export-objs)
change, we will rebuild all $(export-objs) files in the same subdirectory,
which may take a noticable amount of time and thus is annoying during
development, and, of course, unnecessary.

The patch is originally from Russell King and has been tested in the ARM
kernel tree for a long time.

Rules.make

@@ -304,7 +304,8 @@ $(TOPDIR)/include/linux/modversions.h:
 endif # CONFIG_MODVERSIONS
 
 ifneq "$(strip $(export-objs))" ""
-$(export-objs): $(export-objs:.o=.c) $(TOPDIR)/include/linux/modversions.h
+$(export-objs): $(TOPDIR)/include/linux/modversions.h
+$(export-objs): %.o: %.c
 	$(CC) $(CFLAGS) $(EXTRA_CFLAGS) -DKBUILD_BASENAME=$(subst $(comma),_,$(subst -,_,$(*F))) $(CFLAGS_$@) -DEXPORT_SYMTAB -c $(@:.o=.c)
 	@ ( \
 	    echo 'ifeq ($(strip $(subst $(comma),:,$(CFLAGS) $(EXTRA_CFLAGS) $(CFLAGS_$@) -DEXPORT_SYMTAB)),$$(strip $$(subst $$(comma),:,$$(CFLAGS) $$(EXTRA_CFLAGS) $$(CFLAGS_$@) -DEXPORT_SYMTAB)))' ; \