Include config.h before any other header

"config.h" must be included first, because otherwise the compiler
might not see the right value of _FILE_OFFSET_BITS.  We've had this
before; see 705868a8a9.  In this case,
GCC would compute a different address for ‘settings.useSubstitutes’ in
misc.cc because of the off_t in ‘settings’.

Reverts 3854fc9b42.

http://hydra.nixos.org/build/3016700

src/libutil/types.hh

@@ -1,5 +1,7 @@
 #pragma once
 
+#include "config.h"
+
 #include <string>
 #include <list>
 #include <set>