[Fix] rspamd_symcache.h: keep C linkage of lua.h for C++ includers

[Fix] rspamd_symcache.h: keep C linkage of lua.h for C++ includers

The raw lua.h include sat outside extern "C", so any C++ unit that
pulled rspamd_symcache.h before lua/lua_common.h got C++-mangled
declarations of the Lua C API and failed to link. Including
lua_common.h itself is not an option due to the include cycle via
rspamd.h -> cfg_file.h -> rspamd_symcache.h, so wrap lua.h the same
way lua_common.h does. This also lets composites sources include
lua_common.h in the natural position instead of first.