Doc: exclude snippets subdir.

This fixes the qdoc warning:
doc/snippets/code/src_corelib_tools_qcommandlineparser.cpp:85: warning: Hostile character 0x24 in C++ source
and doesn't lead to any loss in the generated documentation.

Snippets are not "source files" for qdoc, they are included by source files.

Change-Id: Ica0169813d71994ff258bf7c48f2fd387d33d772
Reviewed-by: Frederik Gladhorn <frederik.gladhorn@digia.com>
Reviewed-by: Martin Smith <martin.smith@digia.com>