checkasm: Include io.h for isatty, if available

configure does check for isatty, and checkasm properly checks
HAVE_ISATTY, but on some platforms (e.g. WinRT), io.h needs to be
included for isatty to be available.
Signed-off-by: Martin Storsjö <martin@martin.st>

tests/checkasm/checkasm.c

@@ -29,6 +29,10 @@
 #include "libavutil/cpu.h"
 #include "libavutil/random_seed.h"
 
+#if HAVE_IO_H
+#include <io.h>
+#endif
+
 #if ARCH_X86
 #include "libavutil/x86/cpu.h"
 #endif