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

patch by Arvid Picciani (aep)

include/net/ethernet.h

@@ -1,6 +1,10 @@
 #ifndef __NET_ETHERNET_H
 #define __NET_ETHERNET_H 1
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 #include <stdint.h>
 #include <sys/types.h>
 #include <netinet/if_ether.h>
@@ -44,4 +48,8 @@ struct ether_header {
 #define	ETHERMTU	ETH_DATA_LEN
 #define	ETHERMIN	(ETHER_MIN_LEN - ETHER_HDR_LEN - ETHER_CRC_LEN)
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif