crypto: aegis128 - avoid spurious references crypto_aegis128_update_simd

Geert reports that builds where CONFIG_CRYPTO_AEGIS128_SIMD is not set
may still emit references to crypto_aegis128_update_simd(), which
cannot be satisfied and therefore break the build. These references
only exist in functions that can be optimized away, but apparently,
the compiler is not always able to prove this.

So add some explicit checks for CONFIG_CRYPTO_AEGIS128_SIMD to help the
compiler figure this out.

Tested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>