top: only use 'pthread_sigmask' under separate threads

top: only use 'pthread_sigmask' under separate threads

When multi-threading was introduced in the patch shown
below, the former calls to sigprocmask were traded for
a pthread_sigmask call. This was done unconditionally.

As a result, even when those threads weren't enabled a
need to link with libpthread was created. In hindsight
the need should only arise when top is multi-threaded.

This commit will make pthread_sigmask use conditional.

Reference(s):
. 09/2021, separate threads introduced
commit 29f0a674a85bfb92443c56f070956a7dd60bb5f7

Signed-off-by: Jim Warner <james.warner@comcast.net>