(Folly) Fix error when compiling with clang

(Folly) Fix error when compiling with clang

Summary:
folly/Benchmark.cpp:427:9: error: logical not is only applied to the left hand side of this comparison
if (!strcmp(get<1>(benchmarks[i]), "-") == 0) {

Since the `!` binds to `strcmp`, removing both the `!` and the `== 0`
should be equivalent.

Reviewers of diff that introduced this line: D490324

Test Plan: I compiled `tao/client` with/without this diff.

Reviewed By: delong.j@fb.com

FB internal diff: D1196452

Blame Revision: rFBCODE6f5eea5fcd23044a5c579b040bf45d17006b4adf