Revert "[nfc]remove unused variable after pr/81691 (#82578)"

This reverts commit 4d73cbe.

Author: mingmingl-llvm

llvm/lib/ProfileData/InstrProfWriter.cpp

@@ -456,6 +456,7 @@ Error InstrProfWriter::writeImpl(ProfOStream &OS) {
   Header.BinaryIdOffset = 0;
   Header.TemporalProfTracesOffset = 0;
   Header.VTableNamesOffset = 0;
+  int N = sizeof(IndexedInstrProf::Header) / sizeof(uint64_t);
 
   // Only write out the first four fields. We need to remember the offset of the
   // remaining fields to allow back patching later.