net: ipa: declare the page pointer type in "gsi_trans.h"

The second argument to gsi_trans_page_add() is a page pointer.
That declaration is found in header files used by "gsi_trans.h" for
(at least) arm64 and x86 builds, but apparently not for alpha
builds.

Fix this by adding a declaration of struct page to the top of
"gsi_trans.h".

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>