minor simplification refactoring for timefn

`UTIL_getSpanTimeMicro()` can be factored in a generic way,
reducing OS-dependent code.

programs/fileio.c

@@ -28,6 +28,7 @@
 #include <stdio.h>      /* fprintf, open, fdopen, fread, _fileno, stdin, stdout */
 #include <stdlib.h>     /* malloc, free */
 #include <string.h>     /* strcmp, strlen */
+#include <time.h>       /* clock_t, to measure process time */
 #include <fcntl.h>      /* O_WRONLY */
 #include <assert.h>
 #include <errno.h>      /* errno */