[Fuzzer] Fix function prototype in fuzzer::ExternalFunctions. [NFC]

The __sanitizer_acquire_crash_state function has int return type, but the
fuzzer's external function definitions give it bool.

Places where __sanitizer_acquire_crash_state is declared:

  include/sanitizer_common/sanitizer_interface_defs.h
  lib/sanitizer_common/sanitizer_interface_internal.h
  lib/sanitizer_common/sanitizer_common.cc
  lib/fuzzer/FuzzerExtFunctions.def (this is the only bool)

llvm-svn: 353596