fix RPM build errors due to ghosted site.less not existing

    Recognition of file "usr/share/bkr/server/assets/site.less" failed: mode 100777 cannot open `usr/share/bkr/server/assets/site.less' (No such file or directory)

Change-Id: If15f14d2ec8251f00a9d3e3f847428b488e1d3aa