Don't install bits/libc-lock.h or bits/stdio-lock.h.

The bits/libc-lock.h and bits/stdio-lock.h headers are installed, but
not used by any other installed header.

Accordingly, this patch stops these headers from being installed.  A
followup is intended to move these headers out of the bits/ namespace.

Tested for x86_64 (testsuite, and that installed stripped shared
libraries are unchanged by the patch).

	* Makefile (headers): Remove bits/libc-lock.h.
	* libio/Makefile (headers): Remove bits/stdio-lock.h.