[Makefile] Don't include generated dependencies on clean

If we include dependencies in a Makefile for which a target exists
(currently this only affects the manifest.sgx.d targets) then Make will
generate them and then clean will delete them ... so just don't include
them on clean.

Pal/test/Makefile

@@ -63,7 +63,9 @@ else
 $(executables):
 endif
 
+ifeq ($(filter clean,$(MAKECMDGOALS)),)
 -include $(addsuffix .d, $(executables)) $(addsuffix .i.d, $(executables)) $(addsuffix .s.d, $(executables))
+endif
 
 .PHONY: clean
 clean: