fix: properly include pure C headers from C++ code

Header files included from libosmocore may potentially contain
some language constructions allowed in C but not in C++, such
as type casting. Let's add 'extern "C" { ... }' and be safe.

Change-Id: I7197f7b34f30b49d5397506ce9d67cbf0e2cc196

src/gprs_debug.cpp

@@ -18,8 +18,11 @@
  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
  */
 
+extern "C" {
 #include <osmocom/core/utils.h>
 #include <osmocom/core/logging.h>
+}
+
 #include <gprs_debug.h>
 
 /* default categories */

src/gprs_rlcmac_meas.cpp

@@ -17,7 +17,9 @@
  * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA  02111-1307, USA.
  */
 
+extern "C" {
 #include <osmocom/core/timer_compat.h>
+}
 
 #include <gprs_rlcmac.h>
 #include <gprs_debug.h>