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>

drivers/net/ipa/gsi_reg.h

@@ -10,6 +10,10 @@
 
 #include <linux/bits.h>
 
+struct platform_device;
+
+struct gsi;
+
 /**
  * DOC: GSI Registers
  *