tcg/tcg-op.h: Add multiple include guard

The tcg-op.h header was missing the usual guard against multiple
inclusion; add it.

(Spotted by lgtm.com's static analyzer.)

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20181108125256.30986-1-peter.maydell@linaro.org