Move gsm_rlcmac.cpp -> .c

Original file from wireshark.git (packet-gsm_csn1.c) is being built and
maintained as a C file. There's no real need for us to maintain it as a
C++, and doing so will make both files derive over time (as already
happened). Let's keep it as a C compiler (which btw seems to be more
strict) to make it easier to port patches back and forth wireshark.git.

Take the chance to move some declarations we added to csn1.h to be able
to build it out of wireshark. Let's keep those in a separate header file
to ease looking for differences.

Change-Id: I818a8ae947f002d35142f9f5473454cfd80e1830
diff --git a/src/pdch.h b/src/pdch.h
index 821fb90..97b4b23 100644
--- a/src/pdch.h
+++ b/src/pdch.h
@@ -24,9 +24,9 @@
 #ifdef __cplusplus
 extern "C" {
 #include <osmocom/core/linuxlist.h>
+#include "gsm_rlcmac.h"
 }
 
-#include <gsm_rlcmac.h>
 #include <gprs_coding_scheme.h>
 #include <bts.h>
 #endif