build: add workaround for include_directories() sandbox

I tried using a subproject, but Meson will error out when the
subproject calls meson.override_dependency() because the system
libdrm has been picked already.

The workaround fixes this warning:

    ../meson.build:59: WARNING: include_directories sandbox violation!
    The project is trying to access the directory 'subprojects/libdrm/include/drm' which belongs to a different
    subproject. This is a problem as it hardcodes the relative paths of these two projects.
    This makes it impossible to compile the project in any other directory layout and also
    prevents the subproject from changing its own directory layout.

    Instead of poking directly at the internals the subproject should be executed and
    it should set a variable that the caller can then use. Something like:

    # In subproject
    some_dep = declare_dependency(include_directories: include_directories('include'))

    # In subproject wrap file
    [provide]
    some = some_dep

    # In parent project
    some_dep = dependency('some')
    executable(..., dependencies: [some_dep])

    This warning will become a hard error in a future Meson release.