configure: disable librsvg with --enable-headless

librsvg have a long list of dependencies that requires at least
gdk, so X. On debian the headers package depends on cairo too.
Indeed not what we want.

Since comments here says that impress depends on it will need to
eventually disable build of impress.

Change-Id: Ic88f90d03d0a80c1ada9f4224faed9deab2bed52

configure.in

@@ -4063,6 +4063,7 @@ if test "$enable_headless" = "yes"; then
     test_kde4=no
     test_unix_quickstarter=no
     test_cairo=no
+    enable_librsvg=no
 fi
 
 dnl check for cups support