makefile: rename vchan Makefile to not conflict with windows build

rpm_spec/core-dom0.spec

@@ -54,7 +54,7 @@ python -O -m compileall qvm-core qmemman
 make -C restore
 make -C aux-tools
 make -C ../qubes_rpc
-make -C ../vchan
+make -C ../vchan -f Makefile.linux
 make -C ../u2mfn
 make -C ../qrexec
 

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