build: create $(PKG_SYMVERS_DIR) if non-existent

build: create $(PKG_SYMVERS_DIR) if non-existent

Commit 5d76065 moved the creation of the symvers directory to
include/kernel-build.mk. This is fine when building from scratch. But
when unpacking an SDK the directory doesn't exist and because the kernel
won't be built (again) this directory will not be created by the build
system, causing build failure if make tries to copy files into it.

This moves the creation of the symvers directory back into
include/kernel.mk so that the directory is created in any case.

Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>