[memprof] Fix arm-polly-linux builds

arm-polly-linux seems to be failing because we don't include
<unordered_map>.

https://lab.llvm.org/buildbot/#/builders/90/builds/3090

Author: kazutakahirata

llvm/include/llvm/Transforms/Instrumentation/MemProfiler.h

@@ -16,6 +16,8 @@
 #include "llvm/IR/PassManager.h"
 #include "llvm/ProfileData/MemProf.h"
 
+#include <unordered_map>
+
 namespace llvm {
 class Function;
 class IndexedInstrProfReader;