Add missing Build-Depends: perl-xs-dev

Kentaro Hayashirequested to merge
kenhys/rrdtool:perl-xs-dev into master

Closes: #1102789

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1102789

The perl-xs-dev virtual package ensures that the functionality for building Perl extension modules is available.

Signed-off-by: Kentaro Hayashi kenhys@xdump.org

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