On Ubuntu 21.10 (ppc64le) building raid6test with gcc (Ubuntu
11.2.0-7ubuntu2) 11.2.0 fails with the error below.

    gcc -I.. -I ../../../include -g -O2                       \
             -I../../../arch/powerpc/include -DCONFIG_ALTIVEC \
             -c -o vpermxor1.o vpermxor1.c
    vpermxor1.c: In function ‘raid6_vpermxor1_gen_syndrome_real’:
    vpermxor1.c:64:29: error: expected string literal before ‘VPERMXOR’
       64 |   asm(VPERMXOR(%0,%1,%2,%3):"=v"(wq0):"v"(gf_high), "v"(gf_low), "v"(wq0));
          |       ^~~~~~~~
    make: *** [Makefile:58: vpermxor1.o] Error 1

So, include the header asm/ppc-opcode.h defining this macro also when
not building the Linux kernel but only this too.

Cc: Matt Brown <matthew.brown.dev@gmail.com>
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Song Liu <song@kernel.org>