selinux: fix build warning

This patch make selinux_task_prlimit() static since it is not used
anywhere else.
This fix the following build warning:
security/selinux/hooks.c:3981:5: warning: no previous prototype for 'selinux_task_prlimit' [-Wmissing-prototypes]
Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Paul Moore <paul@paul-moore.com>

security/selinux/hooks.c

@@ -3970,8 +3970,8 @@ static int selinux_task_getioprio(struct task_struct *p)
 			    PROCESS__GETSCHED, NULL);
 }
 
-int selinux_task_prlimit(const struct cred *cred, const struct cred *tcred,
-			 unsigned int flags)
+static int selinux_task_prlimit(const struct cred *cred, const struct cred *tcred,
+				unsigned int flags)
 {
 	u32 av = 0;