Core: ensure that ngx_config.h is always included first.

This fixes compilation of various 3rd party modules when nginx is
configured with threads.

src/core/ngx_core.h

@@ -9,6 +9,9 @@
 #define _NGX_CORE_H_INCLUDED_
 
 
+#include <ngx_config.h>
+
+
 typedef struct ngx_module_s      ngx_module_t;
 typedef struct ngx_conf_s        ngx_conf_t;
 typedef struct ngx_cycle_s       ngx_cycle_t;