libredwg: fix missing include with glibc-2.38

https://hydra.nixos.org/build/236161770/nixlog/4/tail

pkgs/development/libraries/libredwg/default.nix

···
   in ''
     # avoid git dependency
     cp ${printVersion} build-aux/git-version-gen
+    # failing to build otherwise since glibc-2.38
+    sed '1i#include <string.h>' -i programs/dwg2SVG.c
   '';
 
   preConfigure = lib.optionalString (stdenv.isDarwin && enablePython) ''