Fix linknamespace parallel test failures.

Having found that with my script to build many glibc variants I could
reproduce the linknamespace test failures in parallel builds (that
various people had previously reported but I hadn't seen myself), I
investigated those failures further.  This patch adds a missing
dependency to those tests.

Tested for x86_64, including the configuration where I saw those
failures and where I don't see them with this patch.

	* conform/Makefile ($(linknamespace-header-tests)): Also depend on
	$(linknamespace-symlists-tests).