Add restat to avbpubkey

This rule may not always update the output file, which causes ninja
to print this warning:

ninja: Missing `restat`? An output file is older than the most recent input:
 output: out/soong/.intermediates/system_other_avbpubkey/android_x86_64_silvermont/system_other_avbpubkey/system_other.avbpubkey
  input: out/host/linux-x86/bin/avbtool

Test: Presubmits
Change-Id: I5fd96ef6f965b3bad52d3c45878d6b72e94aa4ea
diff --git a/etc/avbpubkey.go b/etc/avbpubkey.go
index 3f998d4..dc242ce 100644
--- a/etc/avbpubkey.go
+++ b/etc/avbpubkey.go
@@ -51,6 +51,7 @@
 		Command: `${avbtool} extract_public_key --key ${in} --output ${out}.tmp` +
 			` && ( if cmp -s ${out}.tmp ${out} ; then rm ${out}.tmp ; else mv ${out}.tmp ${out} ; fi )`,
 		CommandDeps: []string{"${avbtool}"},
+		Restat:      true,
 		Description: "Extracting system_other avb key",
 	})