security/botan2: fix build on powerpc64

aes_power8.cpp needs VSX too apart from crypto:
src/lib/block/aes/aes_power8/aes_power8.cpp:43:49: error: use of undeclared identifier 'vec_vsx_ld'

return (Altivec64x2)reverse_vec((Altivec8x16)vec_vsx_ld(0, key));;
                                             ^

src/lib/block/aes/aes_power8/aes_power8.cpp:48:36: error: use of undeclared identifier 'vec_vsx_ld'

return (Altivec64x2)reverse_vec(vec_vsx_ld(0, src));