In case libva-wayland has its headers not installed in default
locations (like /usr/include), the build fails to include "wayland-client.h":
In file included from gstvaapidisplay_wayland.h:27:0,
/usr/include/va/va_wayland.h:31:28: fatal error: wayland-client.h: No such file or directory
As we already passed VA_CLAGS, /usr/include/va/va_wayland.h could be found, but it is
our fault not to instruct the system that we ALSO care for va_wayland. We correctly query
for libva-wayland.pc in configure and use this in other places as well. It is thus only
correct and consequent, to do it also at this spot.