[PowerPC64] strncase_l-power7.c should use strncase_l.c

This is another one where we'll be wanting the base symbols for
powerpc64le rather than just a power7 variant.

	* sysdeps/powerpc/powerpc64/multiarch/strncase_l-power7.c: Include
	string/strncase_l.c, not string/strncase.c.
	(USE_IN_EXTENDED_LOCALE_MODEL): Don't define.
	(libc_hidden_def): Redefine.