[fuzzer][afl] Fix build with GCC

[orivej]

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 *);
      |                              ^~~

[orivej]

If you approve please merge on my behalf.

compiler-rt/lib/fuzzer/afl/afl_driver.cpp

 static volatile char suppress_warning1 = AFL_DEFER_FORKSVR[0];
 
 // Input buffer.
 static const size_t kMaxAflInputSize = 1 << 20;
 static uint8_t AflInputBuf[kMaxAflInputSize];
 
 // Use this optionally defined function to output sanitizer messages even if
 // user asks to close stderr.
-__attribute__((weak)) extern "C" void __sanitizer_set_report_fd(void *);
+extern "C" __attribute__((weak)) void __sanitizer_set_report_fd(void *);
 
 // Keep track of where stderr content is being written to, so that
 // dup_and_close_stderr can use the correct one.
 static FILE *output_file = stderr;
 
 // Experimental feature to use afl_driver without AFL's deferred mode.
 // Needs to run before __afl_auto_init.
 __attribute__((constructor(0))) static void __decide_deferred_forkserver(void) {