fblualib/thrift/Serialization.cpp: avoid shadowing warnings
fblualib/thrift/Serialization.cpp: avoid shadowing warnings

Summary:
Fix warnings exposed by gcc-4.9.x's -Wshadow-compatible-local
(and/or the stricter -Wshadow-local) options.  Note that these
are both less onerous than -Wshadow.
I plan to enable one of them for all of fbcode, soon.

Rename inner (lambda parameter) "L" to "L2".

This avoids the following errors:

  fblualib/thrift/Serialization.cpp:211:45: error: declaration of 'lua_State* L' shadows a parameter [-Werror=shadow-compatible-local]
  fblualib/thrift/Serialization.cpp:200:16: error: shadowed declaration is here [-Werror=shadow-compatible-local]

Reviewed By: Orvid

Differential Revision: D4442751

fbshipit-source-id: 627e4e2bb701098c1a70a12db13c5d7462eb996e

facebook/fblualib,facebook/fblualib