x86: Add missing IS_IN (libc) check to strncmp-sse4_2.S

Was missing to for the multiarch build rtld-strncmp-sse4_2.os was
being built and exporting symbols:

build/glibc/string/rtld-strncmp-sse4_2.os:
0000000000000000 T __strncmp_sse42

Introduced in:

commit 11ffcacb
Author: H.J. Lu <hjl.tools@gmail.com>
Date:   Wed Jun 21 12:10:50 2017 -0700

    x86-64: Implement strcmp family IFUNC selectors in C

(cherry picked from commit 96ac447d)