cmake: dt: Reconfigure when DTS dependencies change

When DT sources change CMake must be run again. CMake is currently
detecting changes to DT source files that we manually specify, but not
sources that are included through use of the preprocessor.

This patch makes the preprocessor output the includes used and adds
them to the 'CMAKE_CONFIGURE_DEPENDS' property.

This fixes #16791.

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