wayland-egl: introduce wayland-egl-backend.pc

File will be installed alongside the backend header.

This way vendor implementations have enough information about the
interface and they can build their backend/driver library accordingly.

Cc: Miguel A. Vico <mvicomoya@nvidia.com>
Cc: James Jones <jajones@nvidia.com>
Cc: Daniel Stone <daniels@collabora.com>
Cc: duncan-roe <duncan_roe@optusnet.com.au>
Cc: Takanari Hayama <taki@igel.co.jp>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Miguel A Vico Moya <mvicomoya@nvidia.com>
Reviewed-by: Arnaud Vrac <avrac@freebox.fr>