powerpc: Fix tiny bug in strncmp.c

A single underscore was omitted in
sysdeps/powerpc/powerpc64/multiarch/strncmp.c, resulting in use of
power8 version of strncmp instead of power9 version, with significant
performance degradation.

	* sysdeps/powerpc/powerpc64/multiarch/strncmp.c: Fix #ifdef.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>