Build-Depends: limit libglusterfs-dev dependency to supported architectures

Michael Prokoprequested to merge
mika/tgt:mika/glusterfs into master

See #1039604, glusterfs is no longer supported on non x64 architectures, accordingly adjust Build-Depends, now that glusterfs v11.0 is in Debian unstable.

Thanks: Patrick Matthäi pmatthaei@debian.org

Closes: #1041999

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