Bug 991976 - Only fail creating .mozconfig.mk once. r=mshal

client.mk

@@ -209,7 +209,7 @@ $(OBJDIR)/.mozconfig.mk: $(FOUND_MOZCONFIG) $(call mkdir_deps,$(OBJDIR)) $(OBJDI
 # Include that makefile so that it is created. This should not actually change
 # the environment since MOZCONFIG_CONTENT, which MOZCONFIG_OUT_LINES derives
 # from, has already been eval'ed.
--include $(OBJDIR)/.mozconfig.mk
+include $(OBJDIR)/.mozconfig.mk
 endif
 
 # Print out any options loaded from mozconfig.