makefile: rename vchan Makefile to not conflict with windows build

rpm_spec/core-vm.spec

@@ -54,7 +54,7 @@ The Qubes core files for installation inside a Qubes VM.
 
 %build
 make -C u2mfn
-make -C vchan
+make -C vchan -f Makefile.linux
 make -C misc
 make -C qubes_rpc
 make -C qrexec