xterm: fix issue with SRC_URI

ftp server not responding.

WARNING: Failed to fetch URL ftp://invisible-island.net/xterm/xterm-303.tgz, attempting MIRRORS if available
ERROR: Fetcher failure: Fetch command failed with exit code 4, no output
ERROR: Function failed: Fetcher failure for URL: 'ftp://invisible-island.net/xterm/xterm-303.tgz

'. Unable to fetch URL from any source.

Signed-off-by: Armin Kuster <akuster808@gmail.com>