Compile code for memory debuging only with DEBUG_MEM

When it's not in use - do not compile this code.
Improves lcov code coverage results for this code a lot :)

libdm/mm/dbg_malloc.c

@@ -15,6 +15,8 @@
 
 #include "dmlib.h"
 
+#ifdef DEBUG_MEM
+
 #include <assert.h>
 #include <stdarg.h>
 
@@ -249,6 +251,8 @@ void dm_bounds_check_debug(void)
 	}
 }
 
+#endif
+
 void *dm_malloc_aux(size_t s, const char *file __attribute__((unused)),
 		    int line __attribute__((unused)))
 {