signon: Fix running of {pre,post}Configure

Commit 0055c6a introduced a new preConfigure hook that sets the right
qmake path. Unfortunately the mkDerivation attributes of signon override
the whole configurePhase, so this hook isn't run at all.

This fixes the build of signon and it now successfully compiles on my
machine.

Signed-off-by: aszlig <[e-mail]>

pkgs/development/libraries/signon/default.nix

···
   nativeBuildInputs = [ doxygen ];
 
   configurePhase = ''
+    runHook preConfigure
     qmake PREFIX=$out LIBDIR=$out/lib CMAKE_CONFIG_PATH=$out/lib/cmake/SignOnQt5
+    runHook postConfigure
   '';
 
 }