kernelsrc.bbclass: Fix externalsrc support

When the externalsrc class is used the tasks listed in
SRCTREECOVEREDTASKS are deleted to prevent them being executed. If
externalsrc is used for the kernel then this will include
virtual/kernel:do_patch.

We can depend on do_shared_workdir instead as this will survive when
externalsrc is used.

Signed-off-by: Paul Barker <pbarker@konsulko.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>