[LibFuzzer] Fix `-Wpedantic` warning reported by Eric Christopher.

The warning is reproducible with GCC 4.8. Thanks to David Blaikie for
the suggested fix.

The reported warning was

```
/usr/local/google/home/echristo/sources/llvm/lib/Fuzzer/FuzzerExtFunctions.def:29:10: warning: ISO C++ forbids casting between pointer-to-function and pointer-to-object [-Wpedantic]
 EXT_FUNC(__lsan_enable, void, (), false);
          ^
/usr/local/google/home/echristo/sources/llvm/lib/Fuzzer/FuzzerExtFunctionsWeak.cpp:44:24: note: in definition of macro ‘EXT_FUNC’
   CheckFnPtr((void *)::NAME, #NAME, WARN);
                        ^
```

Differential Revision: https://reviews.llvm.org/D35243

llvm-svn: 307686