gmtp: pass --datapath in the wrapper

gmtp gets somewhat confused trying to lookup some of its own assets when
installed system-wide. It first attempts to locate the path to its own binary by
searching for itself in $PATH, arrives at /run/current-system/sw/bin/gmtp, and
then performs lookups relative to that path without dereferencing the
symlink. Some of the lookups result in searching for, for example, icons in
/run/current-system/sw/bin/../share/gmtp/ which doesn't exist.

pkgs/applications/misc/gmtp/default.nix

@@ -17,6 +17,10 @@ stdenv.mkDerivation {
 
   enableParallelBuilding = true;
 
+  preFixup = ''
+    gappsWrapperArgs+=(--add-flags "--datapath \"$out/share\"");
+  '';
+
   meta = {
     description = "A simple MP3 and Media player client for UNIX and UNIX like systems.";
     homepage = https://gmtp.sourceforge.io;