powerpc/crypto: fix missing skcipher dependency for aes-gcm-p10

My stripped down configuration fails to build with:

  ERROR: modpost: "skcipher_walk_aead_encrypt" [arch/powerpc/crypto/aes-gcm-p10-crypto.ko] undefined!
  ERROR: modpost: "skcipher_walk_done" [arch/powerpc/crypto/aes-gcm-p10-crypto.ko] undefined!
  ERROR: modpost: "skcipher_walk_aead_decrypt" [arch/powerpc/crypto/aes-gcm-p10-crypto.ko] undefined!

Fix it by selecting CRYPTO_SKCIPHER.

Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/c55ad70799e027a3d2756b85ccadc0af52ae8915.1689007370.git.osandov@osandov.com