Use libc_hidden_* for __cmsg_nxthdr (bug 15105).

Among other localplt test failures when building with -Os, there are
libc.so PLT references for __cmsg_nxthdr.  This is a simple case of a
function that is inlined for -O2 but not for -Os; this patch adds
libc_hidden_proto / libc_hidden_def for it to avoid a localplt failure
even when it is not inlined.

Tested for x86_64 (both that it removes this particular localplt
failure for -Os - but other such failures remain so the bug can't yet
be closed - and that the testsuite continues to pass without -Os).

	[BZ #15105]
	* include/sys/socket.h [!_ISOMAC] (__cmsg_nxthdr): Use
	libc_hidden_proto.
	* sysdeps/unix/sysv/linux/cmsg_nxthdr.c (__cmsg_nxthdr): Use
	libc_hidden_def.

(cherry picked from commit e4452a2d)