gnupg: add option for AES cipher

Similar to our RSA option. Enabling this adds ~17K to the gpg binary (ARMv7):

ls -lah output/target/usr/bin/gpg{.orig,}
-rwxr-xr-x 1 peko peko 532K Jul  2 17:29 output/target/usr/bin/gpg
-rwxr-xr-x 1 peko peko 515K Jul  2 17:29 output/target/usr/bin/gpg.orig

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>