[lint] Add blanket waiver for DECLFILENAME with blackboxes
[lint] Add blanket waiver for DECLFILENAME with blackboxes

When running Verilator lint with blackboxed modules, we get warnings
like the ones below:

```
%Warning-DECLFILENAME: ../../../vendor/lowrisc_ip/prim_xilinx/rtl/prim_xilinx_clock_gating.sv:12:10: Filename 'prim_xilinx_clock_gating' does not match NOTFOUNDMODULE name: 'BUFGCE'
   12 |   BUFGCE u_bufgce (
      |          ^~~~~~~~
                       ... Use "/* verilator lint_off DECLFILENAME */" and lint_on around source to disable this message.
%Warning-DECLFILENAME: ../../../vendor/lowrisc_ip/prim_xilinx/rtl/prim_xilinx_clock_mux2.sv:17:11: Filename 'prim_xilinx_clock_mux2' does not match NOTFOUNDMODULE name: 'BUFGMUX'
   17 |   BUFGMUX bufgmux_i (
      |           ^~~~~~~~~
%Warning-DECLFILENAME: ../../../vendor/lowrisc_ip/prim_xilinx/rtl/prim_xilinx_pad_wrapper.sv:36:9: Filename 'prim_xilinx_pad_wrapper' does not match NOTFOUNDMODULE name: 'IOBUF'
   36 |   IOBUF i_iobuf (
      |         ^~~~~~~
```

This is due to https://github.com/verilator/verilator/issues/2430. Until
the issue is fixed, disable this lint error.

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

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