[fuzzer][afl] Fix build with GCC

Summary:
Fixes this build error with GCC 9.3.0:

```
../lib/fuzzer/afl/afl_driver.cpp:114:30: error: expected unqualified-id before string constant
  114 | __attribute__((weak)) extern "C" void __sanitizer_set_report_fd(void *);
      |                              ^~~
```

Reviewers: metzman, kcc

Reviewed By: kcc

Subscribers: #sanitizers

Tags: #sanitizers

Differential Revision: https://reviews.llvm.org/D80479