cmake: ld: Ensure that a linker script change triggers a rebuild

This fixes
https://github.com/zephyrproject-rtos/zephyr/issues/5010

. CMake has a
simple parser that can parse C-like files to find header
dependencies. But the parser needs to know what the include
directories are to be able to follow #include pragmas.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>