[libFuzzer] Don't use `#ifdef` for defined macros, instead use `#if`.

Differential Revision: https://reviews.llvm.org/D28972

llvm-svn: 292670
diff --git a/llvm/lib/Fuzzer/FuzzerShmemPosix.cpp b/llvm/lib/Fuzzer/FuzzerShmemPosix.cpp
index cc06f33..820d64d 100644
--- a/llvm/lib/Fuzzer/FuzzerShmemPosix.cpp
+++ b/llvm/lib/Fuzzer/FuzzerShmemPosix.cpp
@@ -9,7 +9,7 @@
 // SharedMemoryRegion
 //===----------------------------------------------------------------------===//
 #include "FuzzerDefs.h"
-#ifdef LIBFUZZER_POSIX
+#if LIBFUZZER_POSIX
 
 #include "FuzzerIO.h"
 #include "FuzzerShmem.h"