x86: Add missing IS_IN (libc) check to strcspn-sse4.c

Was missing to for the multiarch build rtld-strcspn-sse4.os was
being built and exporting symbols:

build/glibc/string/rtld-strcspn-sse4.os:
                 U ___m128i_shift_right
                 U __strcspn_generic
0000000000000000 T __strcspn_sse42
                 U strlen

build/glibc/string/rtld-varshift.os:
0000000000000000 R ___m128i_shift_right

Introduced in:

commit 06e51c8f
Author: H.J. Lu <hongjiu.lu@intel.com>
Date:   Fri Jul 3 02:48:56 2009 -0700

    Add SSE4.2 support for strcspn, strpbrk, and strspn on x86-64.