doc: Quell warnings about missing man3 directory before its been built

The shell command for dist_man3_MANS gets invoked several times during
the make process but before the man pages have been generated, which
causes the following warnings when running `make`:

    find: `man/man3': No such file or directory
    find: `man/man3': No such file or directory
    find: `man/man3': No such file or directory
      GEN    xml/client/index.xml

Despite these error messages, the generated dist tarball contains the
man3 pages as intended, both before and after this patch.

    $ make dist
    $ tar xxf wayland-1.5.90.tar.xz
    $ find wayland-1.5.90/doc/doxygen/man/man3 -name "wl_*.3" | wc -l
    85

Signed-off-by: Bryce Harrington <b.harrington@samsung.com>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Tested-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>