build: ensure PKG_INFO_DIR exists before trying to use it

PKG_INFO_DIR is only created at the finish step of the first package
build, but kernel modules now use it at the start. Doing a parallel build
could cause a kernel module to be the first package, breaking the build.

Fix this by ensuring the directory exists.

Fixes: 2e496876c60c ("kernel: collect module symvers for external modules")
Signed-off-by: Jonas Gorski <jonas.gorski@gmail.com>