doc/doxy-wrapper: disable inserting headers and footers

The headers and footers are used to integrate Doxygen into Libav's website.
Fixes the Libav-ish look when doing `make apidoc`.

Signed-off-by: Timothy Gu <timothygu99@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>