spl: fit: select SPL_CRYPTO_SUPPORT for SPL_FIT_SIGNATURE

Verifying FIT images obviously needs the rsa parts of crypto
support and while main uboot always compiles crypto support,
it's optional for SPL and we should thus select the necessary
option to not end up in compile errors like:

    u-boot/lib/rsa/rsa-verify.c:328: undefined reference to `rsa_mod_exp'

So select SPL_CRYPTO_SUPPORT in SPL_FIT_SIGNATURE.

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>