xfce.bbclass: Mark string as raw in a regexp

Fixes python Deprecated behavior warning

xfce.bbclass:3: DeprecationWarning: invalid escape sequence \.
  m = re.match("^([0-9]+)\.([0-9]+)", v)

Signed-off-by: Khem Raj <raj.khem@gmail.com>

meta-xfce/classes/xfce.bbclass

@@ -1,6 +1,6 @@
 def xfce_verdir(v):
     import re
-    m = re.match("^([0-9]+)\.([0-9]+)", v)
+    m = re.match(r"^([0-9]+)\.([0-9]+)", v)
     return "%s.%s" % (m.group(1), m.group(2))
 
 HOMEPAGE = "http://www.xfce.org"