toolchain: kernel-headers: fix check target for kernel Git tree

toolchain: kernel-headers: fix check target for kernel Git tree

Currently the check target fails if the kernel Git tree is used:

 $ make toolchain/kernel-headers/{download,check}

  make[2]: Entering directory 'toolchain/kernel-headers'
  Makefile:105: *** ERROR: Unknown pack format for file openwrt/tmp/dl/.  Stop.
  make[2]: Leaving directory 'toolchain/kernel-headers'
  toolchain/Makefile:100: recipe for target 'toolchain/kernel-headers/check' failed

Signed-off-by: Petr Štetiar <ynezz@true.cz>
(cherry picked from commit bb7ba6b6a81d1fb7ac6075edfd8e8b713dd61db2)