Convert gprs_bssgp_pcu.cpp to C

There's no real use of C++ in that file, and it causes problems when
using llist_head entry macros in future patches adding initial support
for multiple BTS in PCU object, so let's move it to plain C.

Change-Id: Ic771a89fd78b5e66151a5384f0ff6a8895589466
diff --git a/src/rlc.h b/src/rlc.h
index eb79430..187a70e 100644
--- a/src/rlc.h
+++ b/src/rlc.h
@@ -18,13 +18,17 @@
  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
  */
 #pragma once
-
+#ifdef __cplusplus
 extern "C" {
-#include "coding_scheme.h"
-}
+#endif
 
+#include "coding_scheme.h"
 #include <osmocom/core/endian.h>
 
+#ifdef __cplusplus
+}
+#endif
+
 #include <stdint.h>
 #include <string.h>