Hopefully actually fix Windows build.

Change-Id: I916d0aeb23eddc9e4ce88e21d8b395396d84d9db
diff --git a/include/perfetto/base/file_utils.h b/include/perfetto/base/file_utils.h
index 1efb450..71f716f 100644
--- a/include/perfetto/base/file_utils.h
+++ b/include/perfetto/base/file_utils.h
@@ -21,6 +21,8 @@
 
 #include <string>
 
+#include "perfetto/base/utils.h"  // For ssize_t on Windows.
+
 namespace perfetto {
 namespace base {