wireshark: Don't redefine ws_plugindir

autoconf already defines the variable for us, and prints out
a warning if we try to do it a second time. So let's not :)

tools/Makefile.am

@@ -398,7 +398,6 @@ EXTRA_DIST += \
 
 if WITH_WIRESHARK_DISSECTOR
 
-ws_plugindir = @ws_plugindir@
 ws_plugin_LTLIBRARIES = wireshark/src/libvirt.la
 wireshark_src_libvirt_la_CPPFLAGS = \
 	-I wireshark/src $(WIRESHARK_DISSECTOR_CFLAGS)