wireshark: Drop DESTDIR from install path

In 50078cfb I've tried to fix distcheck but accidentally
broke rpm build. The problem is that rpm build not only sets
DESTDIR but also passes plugindir path. This results in double
DESTDIR being in the plugin path, Drop one.
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

tools/Makefile.am

@@ -381,7 +381,7 @@ EXTRA_DIST += \
 
 if WITH_WIRESHARK_DISSECTOR
 
-ws_plugindir = $(DESTDIR)$(plugindir)
+ws_plugindir = $(plugindir)
 ws_plugin_LTLIBRARIES = wireshark/src/libvirt.la
 wireshark_src_libvirt_la_CPPFLAGS = \
 	-I wireshark/src $(WIRESHARK_DISSECTOR_CFLAGS)