Don't install the internal header grp-merge.h

grp-merge.h was introduced in Stephen Gallagher's patch adding the
"group merging" feature to NSS.  It declares two functions, __copy_grp
and __merge_grp, both of which are tagged 'internal_function', which
means that nobody can even compile the contents of the header without
access to libc-symbols.h, which is not installed.  (Also, these
functions are GLIBC_PRIVATE exports from libc.so.)  Hence I believe
grp-merge.h should not be installed either.

This really needs to be in 2.24, so that no released version of the
library installs this header.

I hope that what I did to the ChangeLog diff will allow it to be
applied without hassle.

	* grp/Makefile: Don't install the internal header grp-merge.h.