xz: Avoid a compiler warning in progress_speed() in message.c.

This should be smaller too since it avoids the string constants.

src/xz/message.c

@@ -355,11 +355,8 @@ progress_speed(uint64_t uncompressed_pos, uint64_t elapsed)
 	if (elapsed < 3000)
 		return "";
 
-	static const char unit[][8] = {
-		"KiB/s",
-		"MiB/s",
-		"GiB/s",
-	};
+	// The first character of KiB/s, MiB/s, or GiB/s:
+	static const char unit[] = { 'K', 'M', 'G' };
 
 	size_t unit_index = 0;
 
@@ -381,7 +378,7 @@ progress_speed(uint64_t uncompressed_pos, uint64_t elapsed)
 	//  - 999 KiB/s
 	// Use big enough buffer to hold e.g. a multibyte decimal point.
 	static char buf[16];
-	snprintf(buf, sizeof(buf), "%.*f %s",
+	snprintf(buf, sizeof(buf), "%.*f %ciB/s",
 			speed > 9.9 ? 0 : 1, speed, unit[unit_index]);
 	return buf;
 }