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

patch by Arvid Picciani (aep)

include/sys/swap.h

@@ -1,6 +1,11 @@
 #ifndef _SYS_SWAP_H
 #define _SYS_SWAP_H
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+
 #define	SWAP_FLAG_PREFER        0x8000
 #define	SWAP_FLAG_PRIO_MASK     0x7fff
 #define	SWAP_FLAG_PRIO_SHIFT    0
@@ -8,4 +13,8 @@
 int swapon (const char *, int);
 int swapoff (const char *);
 
+#ifdef __cplusplus
+}
+#endif
+
 #endif