selftests: mptcp: ignore mptcp_diag binary

A new binary is now generated by the MPTCP selftests: mptcp_diag.

Like the other binaries from this directory, there is no need to track
this in Git, it should then be ignored.

Fixes: 00f5e338 ("selftests: mptcp: Add a tool to get specific msk_info")
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250328-net-mptcp-misc-fixes-6-15-v1-4-34161a482a7f@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>