Pointers to two struct types are used in "ipa_gsi.h", without those
struct types being forward-declared.  Add these declarations.

Fixes: c3f398b1 ("soc: qcom: ipa: IPA interface to GSI")
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>