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/protocol.c b/src/protocol.c
index d89603d..7574227 100644
--- a/src/protocol.c
+++ b/src/protocol.c
@@ -1,6 +1,6 @@
 #include <osmocom/core/msgb.h>
-#include "protocol.h"
-#include "diagcmd.h"
+#include "protocol/protocol.h"
+#include "protocol/diagcmd.h"
 
 int diag_push_subsys_hdr(struct msgb *msg, uint8_t subsys, uint8_t code)
 {