Fix build: utf8strvisx() does signed data comparisons, but 'char' type

is unsigned in riscv GCC, so use guaranted signed char type.

Sponsored by:	DARPA, AFRL

usr.bin/top/utils.c

@@ -344,7 +344,7 @@ static const char *vis_encodes[] = {
 int
 utf8strvisx(char *dst, const char *src, size_t src_len)
 {
-	const char *src_p;
+	const signed char *src_p;
 	char *dst_p;
 	int i, j, olen, len;