Revert "Upgrade zerocopy to 0.7.5"

Revert submission 2786927-fmayle-zerocopy-update

Reason for revert: broke build

Reverted changes: /q/submissionid:2786927-fmayle-zerocopy-update

Change-Id: I761aec0fd52826d0ee5fdadc5b99b3df474bd827
diff --git a/pvmfw/src/gpt.rs b/pvmfw/src/gpt.rs
index 71eb569..06bf994 100644
--- a/pvmfw/src/gpt.rs
+++ b/pvmfw/src/gpt.rs
@@ -26,7 +26,6 @@
 use vmbase::util::ceiling_div;
 use vmbase::virtio::{pci, HalImpl};
 use zerocopy::FromBytes;
-use zerocopy::FromZeroes;
 
 type VirtIOBlk = pci::VirtIOBlk<HalImpl>;
 
@@ -157,7 +156,7 @@
 type Lba = u64;
 
 /// Structure as defined in release 2.10 of the UEFI Specification (5.3.2 GPT Header).
-#[derive(FromZeroes, FromBytes)]
+#[derive(FromBytes)]
 #[repr(C, packed)]
 struct Header {
     signature: u64,