waffle: add wayland-protocols when building with wayland

When 'wayland is selected in ' DISTRO_FEATURES, following build error
occurs during do_configure:

| Run-time dependency wayland-protocols found: NO (tried pkgconfig)
|
| ../git/meson.build:120:2: ERROR: Dependency "wayland-protocols" not found, tried pkgconfig

Add "wayland-protocols" to PACKAGECONFIG to resolve missing dependency.

Signed-off-by: Andrey Zhizhikin <andrey.z@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>