Switch to using a static GMutex in the Lua extension code

This fixes the runtime breakage added in r1277.  Using a static
mutex is the currently recommended way, and supported back to
glib 2.32.

git-svn-id: https://svn.code.sf.net/p/aqualung/code/trunk@1281 3cd24cdc-1f22-0410-b8b1-dcf80e670293