rpm-build: use pkg-config to detect wireshark presence

rpm-build: use pkg-config to detect wireshark presence

Wireshark supports pkg-config since 1.11.3.  Right now we build
wireshark-dissectior tool as default trough rpm build only on
fedora >= 21 and there is new wireshark that supports pkg-config.
If someone wants to build libvirt with wireshark-dissector against old
wireshark, they should specify the location by hand.

This patch is mainly to fix wrong dependency on wireshark binary as it
doesn't make sense to require that binary file to just get version info
of that package in makefile.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>