minor: don't redefine macros

the DEBUG macro name and ARRAY_SIZE macro function are frequently
used in other projects. If these projects also use libosmocore,
the macros will be redefined. This also generates a warning message
during compilation.
Not redefining the macros removes the warning message and possible
(but unlikely) mis-redefinition.

Change-Id: I0ba91eae8eacc5542d1647601b372e417ed1713c
diff --git a/include/osmocom/core/logging.h b/include/osmocom/core/logging.h
index 295e5a8..803b4a9 100644
--- a/include/osmocom/core/logging.h
+++ b/include/osmocom/core/logging.h
@@ -16,7 +16,9 @@
 /*! Maximum number of logging filters */
 #define LOG_MAX_FILTERS	8
 
+#ifndef DEBUG
 #define DEBUG
+#endif
 
 #ifdef DEBUG
 /*! Log a debug message through the Osmocom logging framework