soc: qcom: ipa: add pad field to struct ipa_imm_cmd_hw_hdr_init_local

Because of C structure alignment rules, the  ipa_imm_cmd_hw_hdr_init_local
structure consumed 16 bytes, even though only 12 are defined.  Add a pad
field to the end of the structure to make this extra space explicit.

Signed-off-by: Alex Elder <elder@linaro.org>