make busybox more portable

Move some distro specific include files into the appropriate #ifdef
blocks to make the code more portable.

Signed-off-by: Sean MacLennan <seanm@seanm.ca>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>

libbb/missing_syscalls.c

@@ -3,14 +3,13 @@
  *
  * Licensed under GPLv2, see file LICENSE in this source tree.
  */
-
 //kbuild:lib-y += missing_syscalls.o
 
-/*#include <linux/timex.h> - for struct timex, but may collide with <time.h> */
-#include <sys/syscall.h>
 #include "libbb.h"
 
 #if defined(ANDROID) || defined(__ANDROID__)
+/*# include <linux/timex.h> - for struct timex, but may collide with <time.h> */
+# include <sys/syscall.h>
 pid_t getsid(pid_t pid)
 {
 	return syscall(__NR_getsid, pid);