modules/db_mysql: Add function declaration to prevent compiler

warning.

- Define db_mysql_alloc_buffer(void) in km_dbase.h.

modules/db_mysql/km_dbase.h

@@ -125,4 +125,11 @@ int db_insert_update(const db1_con_t* _h, const db_key_t* _k, const db_val_t* _v
 int db_mysql_use_table(db1_con_t* _h, const str* _t);
 
 
+/**
+ * Allocate a buffer for database module
+ * No function should be called before this
+ * \return zero on success, negative value on failure
+ */
+int db_mysql_alloc_buffer(void);
+
 #endif /* KM_DBASE_H */