Add pcu-socket vty config

osmo-bts already supports configuring a different path for the bts<->pcu
socket by using the 'pcu-socket' config field.

Change-Id: I9b3e1171da467519750b201849ec892a1e318129
diff --git a/include/osmocom/pcu/pcuif_proto.h b/include/osmocom/pcu/pcuif_proto.h
index 88dc09e..39103ee 100644
--- a/include/osmocom/pcu/pcuif_proto.h
+++ b/include/osmocom/pcu/pcuif_proto.h
@@ -3,6 +3,8 @@
 
 #include <osmocom/gsm/l1sap.h>
 
+#define PCU_SOCK_DEFAULT	"/tmp/pcu_bts"
+
 #define PCU_IF_VERSION		0x07
 #define TXT_MAX_LEN	128