[lld-macho][nfc] Fix uninitialized members warning from Coverity

We were always assigning to this member before using it, but just to be
safe...

See https://lists.llvm.org/pipermail/llvm-dev/2021-June/151029.html

lld/MachO/InputSection.h

@@ -104,7 +104,7 @@ struct StringPiece {
   uint32_t inSecOff;
   uint32_t hash;
   // Offset from the start of the containing output section.
-  uint64_t outSecOff;
+  uint64_t outSecOff = 0;
 
   StringPiece(uint64_t off, uint32_t hash) : inSecOff(off), hash(hash) {}
 };