kbuild: Ignore __this_module in gen_autoksyms.sh

Module object files can contain an undefined reference to __this_module,
which isn't resolved until we link the final .ko. The kernel doesn't
export this symbol, so ignore it in gen_autoksyms.sh.

Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Tested-by: Steve Muckle <smuckle@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Ramji Jiyani <ramjiyani@google.com>