Send downlink IMMEDIATE ASSIGNMENT on PCH and not on AGCH

The IMSI is used to define paging group on which it is sent.

This is tested with MS that requires correct paging group.
diff --git a/src/pcu_l1_if.h b/src/pcu_l1_if.h
index b8cc5c9..2aac553 100644
--- a/src/pcu_l1_if.h
+++ b/src/pcu_l1_if.h
@@ -36,6 +36,7 @@
 void pcu_l1if_tx_ptcch(msgb *msg, uint8_t trx, uint8_t ts, uint16_t arfcn, 
         uint32_t fn, uint8_t block_nr);
 void pcu_l1if_tx_agch(bitvec * block, int len);
+void pcu_l1if_tx_pch(bitvec * block, int plen, char *imsi);
 
 int pcu_l1if_open(void);
 void pcu_l1if_close(void);