eliminate warning on older versions of gcc

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@9629 65c4cc65-6c06-0410-ace0-fbb531ad65f3

include/asterisk/stringfields.h

@@ -103,7 +103,7 @@ typedef const char * ast_string_field;
   \internal
   \brief A constant empty string used for fields that have no other value
 */
-extern const char *__ast_string_field_empty;
+extern const char __ast_string_field_empty[];
 
 /*!
   \internal

utils.c

@@ -961,7 +961,7 @@ void ast_join(char *s, size_t len, char * const w[])
 	s[ofs] = '\0';
 }
 
-const char const *__ast_string_field_empty = "";
+const char __ast_string_field_empty[] = "";
 
 static int add_string_pool(struct ast_string_field_mgr *mgr, size_t size)
 {