Compile-time guard for iOS7 specific property.

BUG=3487
R=glaznev@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/17969004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@6706 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/modules/video_capture/ios/rtc_video_capture_ios_objc.mm

@@ -43,7 +43,9 @@ using namespace webrtc::videocapturemodule;
     _owner = owner;
     _captureId = captureId;
     _captureSession = [[AVCaptureSession alloc] init];
+#if defined(__IPHONE_7_0) && __IPHONE_OS_VERSION_MAX_ALLOWED >= __IPHONE_7_0
     _captureSession.usesApplicationAudioSession = NO;
+#endif
     _captureChanging = NO;
     _captureChangingCondition = [[NSCondition alloc] init];