doc: add two files and use full path in private_headers

It seems that private_headers doesn't really work because we always do out-of-tree build. This commit makes it use full paths in the headers list and adds two files which causes undefined reference error to it.

It should fix the following error when building with gtk-doc enabled.

Error in gtkdoc helper script:
'gtkdoc-scangobj' failed with status 1
libdazzle-scan.o: In function `get_object_types':
/home/lantw44/gnome/build/libdazzle/doc/libdazzle-scan.c:202: undefined reference to `dzl_menu_button_section_get_type'
/home/lantw44/gnome/build/libdazzle/doc/libdazzle-scan.c:227: undefined reference to `dzl_rect_helper_get_type'
clang: error: linker command failed with exit code 1 (use -v to see invocation)