Allow the interface monitor routines to be called from C++.

Change-Id: I2ae60c1b015a432f5df36bd623f1a4c69b02bef1
Reviewed-on: https://code.wireshark.org/review/3081
Reviewed-by: Guy Harris <guy@alum.mit.edu>

caputils/iface_monitor.h

@@ -22,6 +22,10 @@
 #ifndef IFACE_MONITOR_H
 #define IFACE_MONITOR_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif /* __cplusplus */
+
 #ifdef HAVE_LIBPCAP
 
 /*
@@ -72,4 +76,8 @@ iface_mon_event(void);
 
 #endif /* HAVE_LIBPCAP */
 
+#ifdef __cplusplus
+}
+#endif /* __cplusplus */
+
 #endif /* IFACE_MONITOR_H */