security/botan2: fix build on powerpc64

In file included from src/lib/block/aes/aes_power8/aes_power8.cpp:15:
/usr/lib/clang/18/include/altivec.h:17365:10: error: 'builtin_altivec_crypto_vcipher' needs target feature power8-vector
17365 | return builtin_altivec_crypto_vcipher(a, b);

|          ^