cmake: modules: extension: Fix dts file watch

Fixes an issue in the code that processes the output file of a
compiler to see which files should be watched, the compiler can
combine multiple files into a single line instead of putting them
each on separate lines if the length of the file paths is short,
therefore account for this and split it up into multiple elements

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
(cherry picked from commit f4cfb8cd)