integrity: Fix warning about missing prototypes

integrity: Fix warning about missing prototypes

make W=1 generates the following warning in keyring_handler.c

security/integrity/platform_certs/keyring_handler.c:71:30: warning: no previous prototype for get_handler_for_db [-Wmissing-prototypes]
 __init efi_element_handler_t get_handler_for_db(const efi_guid_t *sig_type)
                              ^~~~~~~~~~~~~~~~~~
security/integrity/platform_certs/keyring_handler.c:82:30: warning: no previous prototype for get_handler_for_dbx [-Wmissing-prototypes]
 __init efi_element_handler_t get_handler_for_dbx(const efi_guid_t *sig_type)
                              ^~~~~~~~~~~~~~~~~~~
Add the missing prototypes by including keyring_handler.h.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Tested-by: Mimi Zohar <zohar@linux.ibm.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>