header file restructuring

Move all protocol definitions to protocol/*.h and keep only
definitions related to our implementation as *.h in the top directory.

Also, remove some header files to reduce header file proliferation.
diff --git a/src/diag_log.c b/src/diag_log.c
index 1bb3a72..2f0bc79 100644
--- a/src/diag_log.c
+++ b/src/diag_log.c
@@ -4,8 +4,8 @@
 #include <osmocom/core/msgb.h>
 
 #include "diag_log.h"
-#include "protocol.h"
-#include "diagcmd.h"
+#include "protocol/protocol.h"
+#include "protocol/diagcmd.h"
 
 /***********************************************************************
  * LOG Configuration / Protocol