logging: allow disabling macros using a new define: LIBOSMOCORE_NO_LOGGING

This was previously unconditionally defined, so embedded targets were
unable to get rid of the log macros and functions.

Change-Id: I589f93d98a6bc5cf6221c56e2fe3f27bfdd200e8
diff --git a/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index a554adc..a95c478 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -15,6 +15,10 @@
 #define DEBUG
 #endif
 
+#ifdef LIBOSMOCORE_NO_LOGGING
+#undef DEBUG
+#endif
+
 #ifdef DEBUG
 /*! Log a debug message through the Osmocom logging framework
  *  \param[in] ss logging subsystem (e.g. \ref DLGLOBAL)