Shrink 'ifeq ($(XPIDL_MODULE),)'-conditional error code from an if-else to just an if, so that gmake clobber, etc. works even when the error would fire.

r=shaver.

config/rules.mk

@@ -1041,7 +1041,7 @@ export:: FORCE
 	@echo "Please define MODULE or XPIDL_MODULE when defining XPIDLSRCS,"
 	@echo "so we have a module name to use when creating MODULE.xpt."
 	@echo; sleep 2; false
-else
+endif
 
 # export .idl files to $(XPDIST)/idl
 $(XPDIST)/idl::
@@ -1082,7 +1082,6 @@ endif
 
 GARBAGE			+= $(XPIDL_GEN_DIR)
 endif
-endif
 
 ################################################################################
 # Generate chrome building rules.