llc: Add missing include directive to llc.h

Currently struct llist_head is used without declaration which
accidently did not produce an error so far.

This commit adds the missing include directive.

Sponsored-by: On-Waves ehf
diff --git a/src/llc.h b/src/llc.h
index a58a65b..80d2f7a 100644
--- a/src/llc.h
+++ b/src/llc.h
@@ -18,6 +18,10 @@
 
 #pragma once
 
+extern "C" {
+	#include <osmocom/core/linuxlist.h>
+}
+
 #include <stdint.h>
 #include <string.h>