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