Fixed link error (multiple symbols) with pj_log_max_level with C++ build

git-svn-id: https://svn.pjsip.org/repos/pjproject/trunk@1061 74dad513-b988-da41-8d7b-12977e46ad98
diff --git a/pjlib/include/pj/log.h b/pjlib/include/pj/log.h
index 4d28abe..3b9e3af 100644
--- a/pjlib/include/pj/log.h
+++ b/pjlib/include/pj/log.h
@@ -177,7 +177,7 @@
 #if 0
 PJ_DECL(int) pj_log_get_level(void);
 #else
-PJ_DECL(int) pj_log_max_level;
+extern int pj_log_max_level;
 #define pj_log_get_level()  pj_log_max_level
 #endif