SELinux: inline selinux_is_enabled in !CONFIG_SECURITY_SELINUX

Without this patch building a kernel emits millions of warning like:

include/linux/selinux.h:92: warning: ?selinux_is_enabled? defined but not used

When it is build without CONFIG_SECURITY_SELINUX.  This is harmless, but
the function should be inlined, so it gets compiled out.
Reported-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>

include/linux/selinux.h

@@ -89,7 +89,7 @@ static inline void selinux_secmark_refcount_dec(void)
 	return;
 }
 
-static bool selinux_is_enabled(void)
+static inline bool selinux_is_enabled(void)
 {
 	return false;
 }