Makefile: use full stem in dependency generation

Previously took only the filename, so the directory
component was stripped. This broke dependencies for
basically every file.

src/Makefile

@@ -376,7 +376,7 @@ ifdef Echo_name
 	@printf '%-20.20s\r' $$<
 endif
 endif
-	$(.)$(cc) -MM -MF $$@ -MT $(objdir)/$$(*F).o $(2) $$<
+	$(.)$(cc) -MM -MF $$@ -MT $(objdir)/$$*.o $(2) $$<
 endef
 
 $(eval $(call _recipe,c))