[LLD][ELF] Silence warning when building with Clang ToT

This fixes:
```
[5839/7452] Building CXX object tools\lld\ELF\CMakeFiles\lldELF.dir\MapFile.cpp.obj
C:\git\llvm-project\lld\ELF\MapFile.cpp(124,38): warning: comparison of integers of different signs: 'uint32_t' (aka 'unsigned int') and 'const ssize_t' (aka 'const int') [-Wsign-compare]
  124 |           last.outputOff + last.size == p.outputOff) {
      |           ~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~
1 warning generated.
```

lld/ELF/MapFile.cpp

@@ -121,7 +121,7 @@ static void printEhFrame(raw_ostream &os, const EhFrameSection *sec) {
     if (!pieces.empty()) {
       EhSectionPiece &last = pieces.back();
       if (last.sec == p.sec && last.inputOff + last.size == p.inputOff &&
-          last.outputOff + last.size == p.outputOff) {
+          last.outputOff + last.size == (unsigned)p.outputOff) {
         last.size += p.size;
         return;
       }