net: ipa: add two missing declarations

When gsi_reg_init() got added, its declaration was added to
"gsi_reg.h" without declaring the two struct pointer types it uses.
Add these struct declarations to "gsi_reg.h".

Fixes: 3c506add

 ("net: ipa: introduce gsi_reg_init()")
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>