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 <rsalz@openssl.org>
(cherry picked from commit b2991c08)