Fix minor portability issue in pg_resetwal.c.

The argument of isspace() (like other <ctype.h> functions)
must be cast to unsigned char to ensure portable results.

Per NetBSD buildfarm members.  Oversight in 636c1914b.

src/bin/pg_resetwal/pg_resetwal.c

@@ -1246,7 +1246,7 @@ strtouint32_strict(const char *restrict s, char **restrict endptr, int base)
 	bool		is_neg;
 
 	/* skip leading whitespace */
-	while (isspace(*s))
+	while (isspace((unsigned char) *s))
 		s++;
 
 	/*