package/tcpdump: remove duplicated binary

Since tcpdump 4.99.0, the 'tcpdump' binary is no longer installed in
/usr/sbin but in /usr/bin. This change invalidates the Buildroot hook
'TCPDUMP_REMOVE_DUPLICATED_BINARY', causing a fairly large rootfs size
increase as a result.

Update the path inside this hook.

Signed-off-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>