[POWERPC] Add strncmp to arch/ppc

Commit 0119536cd314ef95553604208c25bc35581f7f0a added an assembly
version of strncmp to PowerPC.  However, it changed a common header
file between arch/ppc and arch/powerpc without adding strncmp to
arch/ppc.  This fixes that omission so that arch/ppc links again.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>