protocol/meson.build: install content-protection protocol xml

Since this is expected to be used by applications outside of weston
we should install it.

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>

protocol/meson.build

@@ -6,6 +6,7 @@ dir_wp_base = dep_wp.get_pkgconfig_variable('pkgdatadir')
 
 install_data(
 	[
+		'weston-content-protection.xml',
 		'weston-debug.xml',
 		'weston-direct-display.xml',
 	],