osmo-bts-trx: add -x to use the external oscillator

Change-Id: I94ea9ab454d3d73b6d363209282df77d562d45dc
diff --git a/src/osmo_gsm_tester/bts_osmotrx.py b/src/osmo_gsm_tester/bts_osmotrx.py
index 71cdd48..2ea0f0b 100644
--- a/src/osmo_gsm_tester/bts_osmotrx.py
+++ b/src/osmo_gsm_tester/bts_osmotrx.py
@@ -55,7 +55,7 @@
             raise RuntimeError('No lib/ in %r' % self.inst)
         self.env = { 'LD_LIBRARY_PATH': lib }
 
-        self.launch_process(OsmoBtsTrx.BIN_TRX)
+        self.launch_process(OsmoBtsTrx.BIN_TRX, '-x')
         self.launch_process(OsmoBtsTrx.BIN_BTS_TRX, '-r', '1', '-c', os.path.abspath(self.config_file))
         #self.launch_process(OsmoBtsTrx.BIN_PCU, '-r', '1')
         self.suite_run.poll()