libctf: Remove undefined functions from ver. map

Starting with ld.lld-17, ld.lld is invoked with the option
--no-undefined-version enabled by default. Furthermore, The functions
ctf_label_set() and ctf_label_get() are not defined. Their inclusion in
libctf/libctf.ver causes ld.lld-17 to fail emitting the following error
messages:

ld.lld: error: version script assignment of 'LIBCTF_1.0' to symbol 'ctf_label_set' failed: symbol not defined
ld.lld: error: version script assignment of 'LIBCTF_1.0' to symbol 'ctf_label_get' failed: symbol not defined

This patch fixes the issue by removing the symbol names from
libctf/libctf.ver.

[nca: fused in later commit that marked ctf_arc_open as libctf
      only as well.  Added ChangeLog entry.]

Signed-off-by: Nicholas Vinson <nvinson234@gmail.com>

libctf/
	* libctf.ver: drop nonexistent label functions: mark
	ctf_arc_open as libctf-only.