spl: fit: select SPL_HASH_SUPPORT for SPL_FIT_SIGNATURE

rsa-checsum needs support for hash functions or else will run into
compile errors like:
u-boot/lib/rsa/rsa-checksum.c:28: undefined reference to `hash_progressive_lookup_algo'

So similar to the main FIT_SIGNATURE entry selects HASH,
select SPL_HASH_SUPPORT for SPL_FIT_SIGNATURE.

Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
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>