vulkan/wsi: Generate wayland protocol headers separately from EGL

vulkan/wsi: Generate wayland protocol headers separately from EGL

Previously, we were depending on EGL for generating the headers and
providing the protocol symbols. However, since neither Vulkan driver
actually wants to link against EGL, this is kind of pointless. It also
creates a weird build dependency.

v2 [Jason]
 - Add missing wsi/ prefix, MKDIR_GEN

v3 [Emil Velikov]
 - include BUILT_SOURCES/generation rules outside of conditional

Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>