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/utils.h b/include/osmocom/core/utils.h
index f429ba6..0673444 100644
--- a/include/osmocom/core/utils.h
+++ b/include/osmocom/core/utils.h
@@ -14,7 +14,9 @@
  * \file utils.h */
 
 /*! Determine number of elements in an array of static size */
+#ifndef ARRAY_SIZE
 #define ARRAY_SIZE(x) (sizeof(x) / sizeof((x)[0]))
+#endif
 /*! Return the maximum of two specified values */
 #define OSMO_MAX(a, b) ((a) >= (b) ? (a) : (b))
 /*! Return the minimum of two specified values */