[kernel][efi] Make padding explicit in "efi_memory_descriptor".

The struct efi_memory_descriptor has 4 bytes of internal padding
required by the spec. Make the padding explicit to avoid confusion.

Change-Id: I6c516eacbdf95c6ee062381fe2df045e8688ea92
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/441780
Commit-Queue: David Greenaway <dgreenaway@google.com>
Reviewed-by: Joshua Seaton <joshuaseaton@google.com>
Testability-Review: David Greenaway <dgreenaway@google.com>
diff --git a/zircon/kernel/lib/efi/include/efi/boot-services.h b/zircon/kernel/lib/efi/include/efi/boot-services.h
index aef5bad..52b4c0e 100644
--- a/zircon/kernel/lib/efi/include/efi/boot-services.h
+++ b/zircon/kernel/lib/efi/include/efi/boot-services.h
@@ -31,6 +31,7 @@
 
 typedef struct {
   uint32_t Type;
+  uint32_t Padding;
   efi_physical_addr PhysicalStart;
   efi_virtual_addr VirtualStart;
   uint64_t NumberOfPages;