rtp conflicts with current gstclock.h, moving back to experimental

Original commit message from CVS:
rtp conflicts with current gstclock.h, moving back to experimental

configure.ac

@@ -80,10 +80,12 @@ GST_CHECK_FEATURE(EXPERIMENTAL, [enable building of experimental plugins],,
   AC_MSG_WARN(building experimental plugins)
   USE_SMOOTHWAVE="yes"
   USE_TARKIN="yes"
+  USE_RTP="yes"
 ],[
   AC_MSG_NOTICE(not building experimental plugins)
   USE_SMOOTHWAVE="no"
   USE_TARKIN="no"
+  USE_RTP="no"
 ])
 
 dnl broken plugins; stuff that doesn't seem to build at the moment