fix all missing instances of __cplusplus checks/extern "C" in headers

patch by Arvid Picciani (aep)

include/sys/msg.h

@@ -1,6 +1,10 @@
 #ifndef _SYS_MSG_H
 #define _SYS_MSG_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include <sys/ipc.h>
 
 #define __NEED_pid_t
@@ -33,4 +37,8 @@ int msgget (key_t, int);
 ssize_t msgrcv (int, void *, size_t, long, int);
 int msgsnd (int, const void *, size_t, int);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif