openbabel2: fixup build with gcc12

This is actually the one I wanted to fix in commit ad1bdf4e77f6,
but I didn't notice that we have two attributes and expressions.

pkgs/development/libraries/openbabel/2.nix

···
     })
   ];
 
+  postPatch = ''
+    sed '1i#include <ctime>' -i include/openbabel/obutil.h # gcc12
+  '';
+
   buildInputs = [ zlib libxml2 eigen python3 cairo pcre ];
 
   nativeBuildInputs = [ cmake pkg-config ];