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);
       |          ^