Add more headers to include/ for conform tests.

Add more headers to include/ for conform tests.

Carlos reported failures in conform/ tests in environments where the
compiler used could only find headers in glibc's source and build
trees, not any previously installed headers
<https://sourceware.org/ml/libc-alpha/2014-09/msg00040.html>.

This patch adds wrappers for two of the affected headers to include/,
which is the normal way to make headers visible when building or
testing in directories other than the one containing the header (I
suppose these headers weren't needed in any such directories except
conform/, or other build or test failures would have resulted).  I
believe the same issue applies at least to regexp.h and re_comp.h - we
don't currently have conform/ expectations for those, but when such
expectations are added we'll also need to add header wrappers.

Tested for x86_64 (testsuite, and that installed stripped shared
libraries are unchanged by the patch).  I did *not* test a
configuration such as that in which Carlos saw failure.

	* include/cpio.h: New file.
	* include/fmtmsg.h: Likewise.