grgsm_livemon: Set cell_allocation to [0]

Gnuradio 3.8 does not like cell_allocation: '[arfcn.downlink2arfcn(fc)]'
and the Receiver block does not work with an empty cell allocation array

gnuradio.grc.core.platform:["Param - Cell allocation(cell_allocation):\n\tExpression None is invalid for type'float_vector'."]

Change-Id: Iae6de844509b13bdf834c9c7443382b1ab3b0a28
diff --git a/apps/grgsm_livemon.grc b/apps/grgsm_livemon.grc
index fa96cfb..a11b243 100644
--- a/apps/grgsm_livemon.grc
+++ b/apps/grgsm_livemon.grc
@@ -360,7 +360,7 @@
   parameters:
     affinity: ''
     alias: ''
-    cell_allocation: '[arfcn.downlink2arfcn(fc)]'
+    cell_allocation: '[0]'
     comment: ''
     maxoutbuf: '0'
     minoutbuf: '0'