Corrected __libfuzzer_is_present symbol inclusion for MSVC x86 32-bit

The incorrect symbol will cause linking failures for 32-bit targets:

clang_rt.fuzzer-i386.lib(FuzzerDriver.obj) : error LNK2001: unresolved external symbol __libfuzzer_is_present

Verified no longer fails to link with this change for 32-bit and still succeeds for 64-bit MSVC.

Reviewed By: vitalybuka

Differential Revision: https://reviews.llvm.org/D83594