[lld][MachO] Fix -Wsign-compare warning (NFC)

GCC warning:
```
/llvm-project/lld/MachO/InputFiles.cpp:484:24: warning: comparison of integer expressions of different signedness: ‘int64_t’ {aka ‘long int’} and ‘uint64_t’ {aka ‘long unsigned int’} [-Wsign-compare]
484 |           return value < subsectionEntry.offset;
    |                  ~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
```