Add src/rpc/abc.cpp to the files checked by check-rpc-mappings

Summary: Add the Bitcoin-ABC specific RPCs to the check script.

Test Plan:
Should return no error:

./test/lint/check-rpc-mappings.py .

Add the following line to the vRPCConvertParams[] table in
src/rpc/client.cpp:

{"setexcessiveblock", 1, "dummy"},

Then:

./test/lint/check-rpc-mappings.py .

Should trigger an error:

setexcessiveblock argument 1 (named dummy in vRPCConvertParams) is not
defined in dispatch table

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3751