Makefile: Give .ali files an empty recipe

For Ada sources, .ali files are emitted together with their respective
.o files during compilation. To convince `make` that an .ali was updated
when the .o was rebuilt, it needs an empty recipe.

Change-Id: Ie47122ff3d00460600ed1db97362abf68f59b751
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/16639
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

Makefile

@@ -332,7 +332,7 @@ $$(obj)/$(1)/b__$(1).o: $$(obj)/$(1)/b__$(1).adb
 	@printf "    CC         $$(subst $$(obj)/,,$$@)\n"
 	$(CC_$(1)) $$(ADAFLAGS_$(1)) -c -o $$@ $$<
 $(1)-objs += $$(obj)/$(1)/b__$(1).o
-$($(1)-alis): %.ali: %.o
+$($(1)-alis): %.ali: %.o ;
 endef
 
 $(eval $(foreach class,$(filter-out libgnat-%,$(classes)), \