toolchain: fix gcc depends on kernel headers

toolchain: fix gcc depends on kernel headers

GCC needs the kernel headers to compile.
Some GCC file includes asm/unistd.h which is provided by the kernel headers.
Normally the kernel headers build is very fast and ready before the gcc uses
it, but if it clones the kernel from a slow git repository it takes longer
and then it could be that the gcc already wants to use the kernel headers
before they are available. This patch fixes this problem by adding the
missing dependency.

Signed-off-by: Hauke Mehrtens <hauke.mehrtens@intel.com>