cmake: modules: extensions: Fix dts watch file processing

  # the file paths are short, split these up into multiple elements using regex

  string(REGEX REPLACE "([^ ])[ ]([^ ])" "\\1;\\2" input_as_list "${input_as_list}")

  # Pop the first line and treat it specially

  # Pop the first item containing "empty_file.o:"

  list(POP_FRONT input_as_list first_input_line)

  string(FIND ${first_input_line} ": " index)

  math(EXPR j "${index} + 2")

  string(SUBSTRING ${first_input_line} ${j} -1 first_include_file)

  # Remove whitespace before and after filename and convert to CMake path.

  string(STRIP "${first_include_file}" first_include_file)

  file(TO_CMAKE_PATH "${first_include_file}" first_include_file)

  set(result "${first_include_file}")

  # Remove whitespace before and after filename and convert to CMake path.

  foreach(file ${input_as_list})