modes/gcm128.c: fix OPENSSL_SMALL_FOOTPRINT compile failure on affected platforms (PowerPC and AArch64).
modes/gcm128.c: fix OPENSSL_SMALL_FOOTPRINT compile failure
on affected platforms (PowerPC and AArch64).

For reference, minimalistic #ifdef GHASH is sufficient, because
it's never defined with OPENSSL_SMALL_FOOTPRINT and ctx->ghash
is never referred.

Reviewed-by: Rich Salz <c04971a99e5a9ee80eaab4b1deb37e845b0bd697@openssl.org>

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl