ifaddrs: add extern "C"

We need this to be callable from C programs. Thanks to Diver for
reporting!

headers/compatibility/bsd/ifaddrs.h

@@ -6,6 +6,11 @@
 #define _IFADDRS_H
 
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
 struct ifaddrs {
 	struct ifaddrs  *ifa_next;    /* Next item in list */
 	const char      *ifa_name;    /* Name of interface */
@@ -22,4 +27,9 @@ int getifaddrs(struct ifaddrs **ifap);
 void freeifaddrs(struct ifaddrs *ifa);
 
 
+#ifdef __cplusplus
+}
+#endif
+
+
 #endif