[mlir] Fix a warning (NFC)

This patch fixes:

  mlir/lib/Dialect/SparseTensor/IR/SparseTensorDialect.cpp:717:48:
  error: comparison of integers of different signs: 'int64_t' (aka
  'long') and 'uint64_t' (aka 'unsigned long')
  [-Werror,-Wsign-compare]