meson: add header paths to declare_dependency

The `install_headers()` function is used to install the headers present
in `include/<subdirectory>`.

However, when `xorgproto` is used as a subproject, the headers are not
copied and the parent project is not able to find the headers included
in subdirectories.

For example, this error is generated when `xserver` is built with
`xorgproto` as a subproject:

    fatal error: xf86dri.h: No such file or directory
       41 | #include "xf86dri.h"
          |          ^~~~~~~~~~~

This is a known issue present in Meson:
https://github.com/mesonbuild/meson/issues/2546

To workaround the issue, add the required subdirectories to
`declare_dependency()`.

Signed-off-by: José Expósito <jose.exposito89@gmail.com>

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