kernel.bbclass: ensure symlink_kernsrc task gets run even with externalsrc

Commit c5dfc258 (kernel.bbclass: run do_symlink_kernsrc before
do_patch) fixed a race between do_symlink_kernsrc and
do_populate_lic. However, I missed the fact that when
externalsrc.bbclass is in use, the do_patch task doesn't exist,
meaning that do_symlink_kernsrc now doesn't get run at all, breaking
the build.

We need this task to run regardless of whether do_patch exists or not,
so reinstate the configure->symlink_kernsrc dependency explicitly.

Fixes: c5dfc258 (kernel.bbclass: run do_symlink_kernsrc before
do_patch)
Reported-by: Chanho Park <parkch98@gmail.com>

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>