arpd: remove pthread dependency

arpd: remove pthread dependency

Explicit link with pthread is not needed when linking dynamically. Even
static link with recent libdb does not pull in the code that uses
pthread. Finally, the configure check introduced in commit a25df4887d7
(configure: Check for Berkeley DB for arpd compilation) does not add
-lpthread to its link command.

This change allows arpd build with toolchains that do not provide
threads support.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>