.gitignore: add .lua.c suppression in third party

Add missing .gitignore entry for autogenerated .lua.c translation units
located in <third_party/lua> directory.

Follows up #7593

NO_DOC=.gitignore
NO_TEST=.gitignore
NO_CHANGELOG=.gitignore

Signed-off-by: Igor Munkin <imun@tarantool.org>