qemu: enable parallel building

In 3e9186206e, the build, check and install phases were switched from
Ninja to Make, but stdenv only passes `-j` when `enableParallelBuilding`
is set. Due to this, QEMU's Makefile fell back to `-j1` for its Ninja
invocation, leading to very slow, serial builds.

This commit just sets `enableParallelBuilding` to true.

pkgs/by-name/qe/qemu/package.nix

@@ -257,6 +257,7 @@ stdenv.mkDerivation (finalAttrs: {
   dontUseNinjaCheck = true;
   dontUseNinjaInstall = true;
   dontAddStaticConfigureFlags = true;
+  enableParallelBuilding = true;
 
   outputs = [ "out" ] ++ lib.optional enableDocs "doc" ++ lib.optional guestAgentSupport "ga";
   separateDebugInfo = true;