Rpm: fix broken build caused by CMake changes on Fedora 33+

See https://www.spinics.net/lists/fedora-devel/msg272948.html

Author: sedmicha

pkg/rpm/ipfixcol2.spec.in

@@ -45,6 +45,7 @@ IPFIX Collector (ipfixcol).
 
 # Build source code
 %build
+%global _vpath_builddir .
 %cmake .
 make %{?_smp_mflags}