gnucash: Add patch that fixes enableDebugging gnucash

I wouldn't be able to say exactly why this is the case, but with the
flags that enableDebugging adds to the gnucash derivation, the build
fails. The patch added fixes the warning that causes the build to fail
and makes enableDebugging gnucash work.

pkgs/applications/office/gnucash/default.nix

@@ -33,6 +33,15 @@ stdenv.mkDerivation rec {
     sha256 = "0csp8iddhc901vv09gl5lj970g6ili696vwj4vdpkiprp7gh26r5";
   };
 
+  patches = [
+    # Fixes a warning about an initialized variable that kills enableDebugging gnucash builds on nix.
+    # This will most likely be part of the 4.7 release, it will be safe to remove then.
+    (fetchpatch {
+      url = "https://github.com/Gnucash/gnucash/commit/b42052464ba9701a3d1834fc58fa0deb32ab9afe.patch";
+      sha256 = "092957c8jqj4v70fv0ia1wpgl6x34hbwjrichxfbk5ja8l6535gc";
+    })
+  ];
+
   nativeBuildInputs = [ pkg-config makeWrapper cmake gtest swig ];
 
   buildInputs = [