gnu: ppsspp: Unbundle more sources.

* gnu/packages/emulators (ppsspp)[source]: Update unbundling snippet.
[inputs]: Add cityhash, libzip, vulkan-headers, vulkan-loader, xxhash.
[arguments]<#:configure-flags>: Add -DUSE_SYSTEM_LIBZIP=ON.
Update comments accordingly.

Signed-off-by: Nicolas Goaziou <mail@nicolasgoaziou.fr>