Convert osmo_bts_sock.cpp to C

There's no real point in using C++ there, and using C++ makes the
compiler fail to use llist_head in multi-bts patches added later due to:
"""
'offsetof' within non-standard-layout type is conditionally-supported
"""

Change-Id: I8965b5cc5a713e64788b5b6aa183d3035341ddbb
diff --git a/src/pdch.h b/src/pdch.h
index 57e0f43..8871986 100644
--- a/src/pdch.h
+++ b/src/pdch.h
@@ -188,6 +188,7 @@
 extern "C" {
 #endif
 void pdch_free_all_tbf(struct gprs_rlcmac_pdch *pdch);
+void pdch_disable(struct gprs_rlcmac_pdch *pdch);
 #ifdef __cplusplus
 }
 #endif