tests: rlcmac: Fix C vs C++ linkage of extern symbol

RLCMACTest.cpp:31:30: error: conflicting declaration of ‘const log_info gprs_log_info’ with ‘C’ linkage
   31 | extern const struct log_info gprs_log_info;
gprs_debug.h:54:30: note: previous declaration with ‘C++’ linkag

Change-Id: I5922950dd0057bf7eb8578e2144f127082323fc6

tests/rlcmac/RLCMACTest.cpp

@@ -28,7 +28,6 @@
 #include "decoding.h"
 
 extern "C" {
-extern const struct log_info gprs_log_info;
 #include "pcu_vty.h"
 #include <osmocom/vty/telnet_interface.h>
 #include <osmocom/vty/logging.h>
@@ -41,6 +40,8 @@ extern const struct log_info gprs_log_info;
 }
 using namespace std;
 
+#include "gprs_debug.h"
+
 void *tall_pcu_ctx;
 
 void printSizeofRLCMAC()