Added missing define guard to sleep.h

BUG=
TEST=

Review URL: https://webrtc-codereview.appspot.com/656006

git-svn-id: http://webrtc.googlecode.com/svn/trunk@2415 4adac7df-926f-26a2-2b94-8c16560cd09d

src/system_wrappers/interface/sleep.h

@@ -9,6 +9,9 @@
  */
 // An OS-independent sleep function.
 
+#ifndef WEBRTC_SYSTEM_WRAPPERS_INTERFACE_SLEEP_H_
+#define WEBRTC_SYSTEM_WRAPPERS_INTERFACE_SLEEP_H_
+
 namespace webrtc {
 
 // This function sleeps for the specified number of milliseconds.
@@ -17,3 +20,5 @@ namespace webrtc {
 void SleepMs(int msecs);
 
 }  // namespace webrtc
+
+#endif  // WEBRTC_SYSTEM_WRAPPERS_INTERFACE_SLEEP_H_