Exclude fuzz directory from code coverage report

Exclude fuzz directory from code coverage report

gcovr fails to process fuzz/old.gcda and fuzz/old.gcno files after !7045
MR with:

    (WARNING) GCOV produced the following errors processing /builds/isc-projects/bind9/fuzz/old.gcda:
    	Cannot open source file ../../fuzz/old.c
    Cannot open source file ../../lib/dns/include/dns/compress.h
    Cannot open source file ../../lib/isc/include/isc/buffer.h
    ...
    (gcovr could not infer a working directory that resolved it.)

Given that code coverage inspection is meant only for BIND 9 code and
not its tests and auxiliary tools, the "fuzz" directory should be
excluded from being included in the code coverage report.