Bump wayland-protocols dependency to 1.19

Because we weren't properly pinning the wayland-protocols version, and I
can't read, we missed updating this in !563.

Signed-off-by: Daniel Stone <daniels@collabora.com>

protocol/meson.build

 dep_scanner = dependency('wayland-scanner', native: true)
 prog_scanner = find_program(dep_scanner.get_pkgconfig_variable('wayland_scanner'))
 
-dep_wp = dependency('wayland-protocols', version: '>= 1.18')
+dep_wp = dependency('wayland-protocols', version: '>= 1.19')
 dir_wp_base = dep_wp.get_pkgconfig_variable('pkgdatadir')
 
 install_data(