fuzz: fix redeclaration

[holiman]

Fixes an error which prevents the codecov on oss-fuzz from building

+ compile_go_fuzzer github.com/holiman/uint256 Fuzz uint256Fuzz
/usr/local/bin/compile_go_fuzzer: line 28: cd: /root/go/src/github.com/holiman/uint256: No such file or directory
github.com/holiman/uint256
github.com/holiman/uint256
# github.com/holiman/uint256 [github.com/holiman/uint256.test]
./uint256_test.go:1105:6: bigAddMod redeclared in this block
	./fuzz.go:269: other declaration of bigAddMod
./uint256_test.go:1112:6: bigMulMod redeclared in this block
	./fuzz.go:261: other declaration of bigMulMod
ERROR:root:Building fuzzers failed.
Building project uint256
Running command: python /home/runner/work/oss-fuzz/oss-fuzz/infra/helper.py build_fuzzers uint256 --engine libfuzzer --sanitizer coverage --architecture x86_64