gede: replace build.patch with inline sed call

The patch file is much larger than the logical change made.

pkgs/development/tools/misc/gede/build.patch

@@ -1,11 +0,0 @@
---- a/build.py	2017-01-16 21:12:43.000000000 +0100
-+++ b/build.py	2017-02-26 22:03:11.394625315 +0100
-@@ -71,7 +71,7 @@
-         if do_build:
-             if not os.path.exists("Makefile"):
-                 print("Generating makefile")
--                if subprocess.call(['qmake-qt4']):
-+                if subprocess.call(['qmake']):
-                     exit(1)
- 
-             print("Compiling (please wait)")

pkgs/development/tools/misc/gede/default.nix

@@ -10,7 +10,10 @@ stdenv.mkDerivation rec {
   };
 
   buildInputs = [ ctags qt4 python ];
-  patches = [ ./build.patch ];
+
+  postPatch = ''
+    sed -i build.py -e 's,qmake-qt4,qmake,'
+  '';
 
   unpackPhase = ''
     tar xf ${src}