certs: Convert spaces in certs/Makefile to a tab

There's a rule in certs/Makefile for which the command begins with eight
spaces.  This results in:

        ../certs/Makefile:21: FORCE prerequisite is missing
        ../certs/Makefile:21: *** missing separator.  Stop.

Fix this by turning the spaces into a tab.

Fixes: addf4663

 ("certs: Check that builtin blacklist hashes are valid")
Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Mickaël Salaün <mic@linux.microsoft.com>
cc: keyrings@vger.kernel.org
Link: https://lore.kernel.org/r/486b1b80-9932-aab6-138d-434c541c934a@digikod.net/ # v1
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>