Avoid copiler warning at sprintf

PipeHelperWindows.c:15:5: warning: implicit declaration of function
'sprintf' [-Wimplicit-function-declaration]
     sprintf( name, "\\\\.\\Pipe\\pipeHelper-%u-%i",
          ^~~~~~~

spec/ffi/fixtures/PipeHelperWindows.c

@@ -6,6 +6,7 @@
 
 #ifdef _WIN32
 #include <windows.h>
+#include <stdio.h>
 #include "PipeHelper.h"
 
 int pipeHelperCreatePipe(FD_TYPE pipefd[2])