powerpc/altivec: Add missing prototypes for altivec

Some functions prototypes were missing for the non-altivec code. Add the
missing prototypes in a new header file, fix warnings treated as errors
with W=1:

  arch/powerpc/lib/xor_vmx_glue.c:18:6: error: no previous prototype for ‘xor_altivec_2’ [-Werror=missing-prototypes]
  arch/powerpc/lib/xor_vmx_glue.c:29:6: error: no previous prototype for ‘xor_altivec_3’ [-Werror=missing-prototypes]
  arch/powerpc/lib/xor_vmx_glue.c:40:6: error: no previous prototype for ‘xor_altivec_4’ [-Werror=missing-prototypes]
  arch/powerpc/lib/xor_vmx_glue.c:52:6: error: no previous prototype for ‘xor_altivec_5’ [-Werror=missing-prototypes]

The prototypes were already present in <asm/xor.h> but this header file is
meant to be included after <include/linux/raid/xor.h>. Trying to re-use
<asm/xor.h> directly would lead to warnings such as:

  arch/powerpc/include/asm/xor.h:39:15: error: variable ‘xor_block_altivec’ has initializer but incomplete type

Trying to re-use <asm/xor.h> after <include/linux/raid/xor.h> in
xor_vmx_glue.c would in turn trigger the following warnings:

  include/asm-generic/xor.h:688:34: error: ‘xor_block_32regs’ defined but not used [-Werror=unused-variable]

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>