x11vnc: Fix more hard-coded /bin/su

pkgs/tools/X11/x11vnc/default.nix

···
         --replace '"/bin/true"' '"${coreutils}/bin/true"'
         
     substituteInPlace x11vnc/ssltools.h \
+        --replace /bin/su /var/setuid-wrappers/su \
         --replace xdpyinfo ${xorg.xdpyinfo}/bin/xdpyinfo
   '';