fix: device_kernel_headers can't be found in the make world

Since device_kernel_headers is marked as 'vendor:true', it is exported
to the make world as device_kernel_headers.vendor. Use the correct name
with the .vendor suffix.

Bug: none
Test: BOARD_VNDK_VERSION=current m -j does not complain about
device_kernel_headers.

Change-Id: If3eaa3c5832820c914ef427668d70fa8d8d0bf97