gtk-doc.bbclass: Run gtkdocize in ${S} not ${B}

Otherwise it will fail if these two directories are not the same.

(From OE-Core rev: 491823fdc65d124093f1fed5a56173917443e1d6)

Signed-off-by: Phil Blundell <pb@pbcl.net>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/classes/gtk-doc.bbclass

@@ -15,5 +15,5 @@ EXTRA_OECONF_append = "\
 "
 
 do_configure_prepend () {
-	gtkdocize
+	( cd ${S} && gtkdocize )
 }