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:

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

cc @ofourdan @daenzer

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