[LPeg] Missing symbol visibility control

[Dymphna]

Hello,

I notice LPeg does not control its exported API symbols, which causes
issues when building with -fvisibility=hidden (and potentially LTO). This breaks the shared lib.

I've been patching this downstream by adding an LPEG_API macro similar
to LUAMOD_API/LUALIB_API. Would be nice to have it fixed upstream; attached the patch in case it's useful.

--D