Fix "warning: comparison is always false due to limited range of data type"

on platforms with unsigned chars. The comparison in question is there to
determine whether chars are unsigned or not and is based on comparing a
char, initialized to -1, for less than 0. Change the comparison to check
for geater than 0 instead...

bin/sh/mksyntax.c

@@ -139,10 +139,7 @@ main(int argc __unused, char **argv __unused)
 
 	/* Determine the characteristics of chars. */
 	c = -1;
-	if (c < 0)
-		sign = 1;
-	else
-		sign = 0;
+	sign = (c > 0) ? 0 : 1;
 	for (nbits = 1 ; ; nbits++) {
 		d = (1 << nbits) - 1;
 		if (d == c)