[llvm-exegesis] Add missing initializer.

This is a better fix than rL345245.

llvm-svn: 345246
diff --git a/llvm/tools/llvm-exegesis/lib/Target.cpp b/llvm/tools/llvm-exegesis/lib/Target.cpp
index 588c40e..0655777 100644
--- a/llvm/tools/llvm-exegesis/lib/Target.cpp
+++ b/llvm/tools/llvm-exegesis/lib/Target.cpp
@@ -87,7 +87,7 @@
 
 static_assert(std::is_pod<PfmCountersInfo>::value,
               "We shouldn't have dynamic initialization here");
-const PfmCountersInfo PfmCountersInfo::Default = {nullptr, nullptr, nullptr};
+const PfmCountersInfo PfmCountersInfo::Default = {nullptr, nullptr, nullptr, 0u};
 
 const PfmCountersInfo &
 ExegesisTarget::getPfmCounters(llvm::StringRef CpuName) const {