add port io functions to sys/io.h

based on proposal by Isaac Dunham. nonexistance of bits/io.h will
cause inclusion of sys/io.h to produce an error on archs that are not
supposed to have it. this is probably the desired behavior, but the
error message may be a bit unusual.

src/linux/ioperm.c

@@ -1,7 +1,8 @@
-#include <sys/io.h>
 #include "syscall.h"
 
 #ifdef SYS_ioperm
+#include <sys/io.h>
+
 int ioperm(unsigned long from, unsigned long num, int turn_on)
 {
 	return syscall(SYS_ioperm, from, num, turn_on);

src/linux/iopl.c

@@ -1,7 +1,8 @@
-#include <sys/io.h>
 #include "syscall.h"
 
 #ifdef SYS_iopl
+#include <sys/io.h>
+
 int iopl(int level)
 {
 	return syscall(SYS_iopl, level);