llc: Add missing declarations to llc.h

Currently llc.h relies on the structs BTS, timeval, and msgb being
declared before the file is included.

This commit adds forward declaration for these structs, because they
will only be used for pointer types.

Sponsored-by: On-Waves ehf
diff --git a/src/llc.h b/src/llc.h
index 251712a..a58a65b 100644
--- a/src/llc.h
+++ b/src/llc.h
@@ -23,6 +23,10 @@
 
 #define LLC_MAX_LEN 1543
 
+struct BTS;
+struct timeval;
+struct msgb;
+
 /**
  * I represent the LLC data to a MS
  */