Description of problem:
In fact all these dependencies are needed to xfce panel headers from
xfce4-panel package. It's better to separate these headers to
xfce4-panel-devel, since they required by all xfce4 panel plugins.
Version-Release number of selected component (if applicable):
Steps to Reproduce:
1.rpm -e libxml2-devel libxfce4mcs-devel libxfcegui4-devel
2.rpmbuild -ba xfce4-systray-4.0.6-1.src.rpm
3.Wait for build error
it's fixed in xfce4-systray-4.0.6-2. Thanks for your report.