Fix Doxygen file location

Looks like webos_config_build_doxygen is pretty strict about file
location and format. Currently it expect it to be in
${doc_dir}/${doxyfile}.in . So either first argument should be "." or
file should be in doc folder. Without that next error produced:

-- Configuring incomplete, errors occurred!
CMake Error: File /var/tmp/paludis/build/webOS-luna-webkit-api-scm/work/luna-webkit-api-scm/doc/Doxyfile.in does not exist.
CMake Error at /usr/share/cmake/Modules/webOS/webOS.cmake:776 (configure_file):
  configure_file Problem configuring file
Call Stack (most recent call first):
  /usr/share/cmake/Modules/webOS/webOS.cmake:850 (_webos_add_doc_target)
  CMakeLists.txt:27 (webos_config_build_doxygen)

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <norluk@luxoft.com>
Change-Id: I04e2812b95abd3b92eb7244b81f69c0f517a4745