doc: Remove display of WL_PRINTF attribute

Doxygen truncates a WL_PRINTF function attribute, and there does not
seem to be any workaround[1]. When using the attribute like this:

typedef void (*wl_log_func_t)(const char *, va_list) WL_PRINTF(1, 0);

Doxygen generates something that looks like this:

typedef void (*wl_log_func_t)(const char *, va_list) WL_PRINTF(1,

Configure doxygen to consider WL_PRINTF(x,y) as predefined, so it does
not display the attribute at all in the generated documentation.

[1] https://bugzilla.gnome.org/show_bug.cgi?id=774741



Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>