kbuild: create include/generated in silentoldconfig

The toplevel Makefile creates the directory if it runs silentoldconfig
automatically, but if run manually, it fails:

  $ make mrproper
  $ make defconfig && make silentoldconfig
  *** Default configuration is based on 'x86_64_defconfig'
  #
  # configuration written to .config
  #
  scripts/kconfig/conf -s arch/x86/Kconfig

  *** Error during update of the kernel configuration.
  ...

Move the mkdir command to the silentoldconfig target to make it work.

Signed-off-by: Michal Marek <mmarek@suse.cz>