wireshark: Hoist $ws_prefix declaration

Keep all variable declarations close together.

m4/virt-wireshark.m4

@@ -29,13 +29,13 @@ AC_DEFUN([LIBVIRT_CHECK_WIRESHARK],[
   if test "x$with_wireshark_dissector" != "xno" ; then
     if test "x$with_ws_plugindir" = "xcheck" ; then
       plugindir="$($PKG_CONFIG --variable plugindir wireshark)"
+      ws_prefix="$($PKG_CONFIG --variable prefix wireshark)"
       ws_modversion="$($PKG_CONFIG --modversion wireshark)"
       if test "x$plugindir" = "x" ; then
         dnl On some systems the plugindir variable may not be stored within pkg config.
         dnl Fall back to older style of constructing the plugin dir path.
         plugindir="$libdir/wireshark/plugins/$ws_modversion"
       else
-        ws_prefix="$($PKG_CONFIG --variable prefix wireshark)"
         if test "x$ws_prefix" = "x" ; then
           ws_prefix="/usr";
         fi