semihosting: Move common-semi.h to include/semihosting/

This header is not private to the top-level semihosting directory,
so place it in the public include directory.

Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

diff --git a/semihosting/common-semi.h b/include/semihosting/common-semi.h
similarity index 100%
rename from semihosting/common-semi.h
rename to include/semihosting/common-semi.h