Doxygen: respect code indentation

With this commit, when viewing the source code of a file in the Trinity API docs
(e.g. http://trinitydesktop.org/docs/trinity/tdelibs/tdeui/html/kkeydialog_8cpp_source.html)
the line is correctly indented by using the `white-space: pre;` CSS rule.

Signed-off-by: Mavridis Philippe <mavridisf@gmail.com>
(cherry picked from commit e11968425d)

doc/common/doxygen.css

@@ -98,6 +98,7 @@ SPAN.charliteral   { color: #008080 }
 .line {
 text-align: left;
 font-family: monospace;
+white-space: pre;
 }
 
 .lineno {