sflow: Include the windefs.h header for windows.

sflow.h uses u_int32_t that needs windefs.h for compilation
in visual studio.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>

include/windows/windefs.h

@@ -23,6 +23,7 @@
 #include <windows.h>
 #include <BaseTsd.h>
 #include <io.h>
+#include <inttypes.h>
 
 #pragma comment(lib, "advapi32")
 

lib/sflow.h

@@ -8,6 +8,10 @@
 #ifndef SFLOW_H
 #define SFLOW_H 1
 
+#ifdef _WIN32
+#include "windefs.h"
+#endif
+
 typedef enum {
     SFL_DSCLASS_IFINDEX = 0,
     SFL_DSCLASS_VLAN = 1,