[mlir] Fix warnings
This patch fixes:
mlir/lib/Dialect/OpenMP/IR/OpenMPDialect.cpp:3047:16: error: unused
variable 'ctx' [-Werror,-Wunused-variable]
mlir/lib/Dialect/OpenMP/IR/OpenMPDialect.cpp:3171:16: error: unused
variable 'ctx' [-Werror,-Wunused-variable]
