fpga: xilinx-pr-decoupler: Address warning about unused variable

warning: ‘xlnx_pr_decoupler_of_match’ defined but not used
 [-Wunused-const-variable=]
 static const struct of_device_id xlnx_pr_decoupler_of_match[] = {

Fixes: 7e961c12 ("fpga: Add support for Xilinx LogiCORE PR Decoupler")
Cc: Tom Rix <trix@redhat.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>