IMA: remove -Wmissing-prototypes warning

With W=1 build, the compiler throws warning message as below:

   security/integrity/ima/ima_mok.c:24:12: warning:
   no previous prototype for ‘ima_mok_init’ [-Wmissing-prototypes]
       __init int ima_mok_init(void)

Silence the warning by adding static keyword to ima_mok_init().

Signed-off-by: Austin Kim <austin.kim@lge.com>
Fixes: 41c89b64d7 ("IMA: create machine owner and blacklist keyrings")
Cc: stable@vger.kernel.org
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>

security/integrity/ima/ima_mok.c

@@ -21,7 +21,7 @@ struct key *ima_blacklist_keyring;
 /*
  * Allocate the IMA blacklist keyring
  */
-__init int ima_mok_init(void)
+static __init int ima_mok_init(void)
 {
 	struct key_restriction *restriction;