Revert "x86emul/test: drop bogus .PHONY"

This reverts commit 24c471b6f4. I
wasn't thinking quite right; the line was added just recently,
when the symlink-ed dir was converted to a real one. Thanks to
Athony Perard for noticing the mistake.

tools/tests/x86_emulator/Makefile

@@ -278,6 +278,7 @@ else
 run32 clean32: %32: %
 endif
 
+.PHONY: x86_emulate
 x86_emulate:
 	mkdir -p $@
 	ln -sf $(XEN_ROOT)/xen/arch/x86/$@/*.[ch] $@/