Fix capture on linux (#35)

* disable desktop source on linux

* fix bug:In Win32, the header file is referenced, but RTC_DESKTOP_DEVICE is not defined

Author: zjzhang-cn

include/rtc_types.h

@@ -11,6 +11,10 @@
 #define LIB_WEBRTC_API
 #endif
 
+#if !defined(RTC_DESKTOP_DEVICE) && defined(WIN32)
+#define RTC_DESKTOP_DEVICE
+#endif
+
 #include "base/fixed_size_function.h"
 #include "base/portable.h"
 #include "base/refcount.h"