Convert gprs_bssgp_pcu.cpp to C

There's no real use of C++ in that file, and it causes problems when
using llist_head entry macros in future patches adding initial support
for multiple BTS in PCU object, so let's move it to plain C.

Change-Id: Ic771a89fd78b5e66151a5384f0ff6a8895589466
diff --git a/src/gprs_bssgp_pcu.h b/src/gprs_bssgp_pcu.h
index 080893f..df22541 100644
--- a/src/gprs_bssgp_pcu.h
+++ b/src/gprs_bssgp_pcu.h
@@ -21,7 +21,6 @@
 #ifndef GPRS_BSSGP_PCU_H
 #define GPRS_BSSGP_PCU_H
 
-
 #ifdef __cplusplus
 extern "C" {
 #endif
@@ -40,6 +39,7 @@
 #include <gprs_debug.h>
 
 #include <time.h>
+#include <unistd.h>
 
 #define QOS_PROFILE 4
 #define BSSGP_HDR_LEN 53