fix 1988/spinellis

Prevent the Makefile from hanging on stdin by
providing some sample input files and asking the
user to run the command themselves.

1988/spinellis/input.txt

@@ -0,0 +1,8 @@
+#include <stdio.h>
+
+int
+main(void)
+{
+    printf("Hello, world!\n");
+    return 0;
+}