GSM: fix build if mbed trace is disabled

Author: pennam

libraries/GSM/src/GSM.h

@@ -108,8 +108,10 @@ class GSMClass : public MbedSocketClass {
   bool setTime(unsigned long const epoch, int const timezone = 0);
   void enableCmux();
   bool isCmuxEnable();
+#if MBED_CONF_MBED_TRACE_ENABLE
   void trace(Stream& stream);
   void setTraceLevel(int trace_level, bool timestamp = false);
+#endif
   int ping(const char* hostname, uint8_t ttl = 128);
   int ping(const String& hostname, uint8_t ttl = 128);
   int ping(IPAddress host, uint8_t ttl = 128);