virt-wireshark: Properly substract wireshark prefix

So, when building wireshark plugin, we get the plugindir variable
from the wireshark.pc as well as prefix. Then we replace the
prefix in the plugindir with our own prefix where libvirt is
building to:

  plugindir="${prefix}${plugindir#ws_prefix}"

However, as you can see, there's '$' missing in front of the
ws_prefix variable. This results in the mangled plugindir, for
instance like this:

  plugindir='/usr/usr/lib64/wireshark/plugins'
Signed-off-by: NMichal Privoznik <mprivozn@redhat.com>

m4/virt-wireshark.m4

@@ -38,7 +38,7 @@ AC_DEFUN([LIBVIRT_CHECK_WIRESHARK],[
         if test "x$ws_prefix" = "x" ; then
           ws_prefix="/usr";
         fi
-        plugindir="${prefix}${plugindir#ws_prefix}"
+        plugindir="${prefix}${plugindir#$ws_prefix}"
       fi
     elif test "x$with_ws_plugindir" = "xno" || test "x$with_ws_plugindir" = "xyes"; then
       AC_MSG_ERROR([ws-plugindir must be used only with valid path])