gnu: prosody: Update to 0.9.12.

* gnu/packages/messaging.scm (prosody): Update to 0.9.12.
  [inputs]: Change lua5.1-sec-0.5 to lua5.1-sec.
* gnu/packages/lua.scm (lua5.1-sec-0.5): Remove variable.

Signed-off-by: Marius Bakke <mbakke@fastmail.com>