[libFuzzer] make libFuzzer build even with a compiler that does not have sanitizer headers

llvm-svn: 253003
diff --git a/llvm/lib/Fuzzer/FuzzerIO.cpp b/llvm/lib/Fuzzer/FuzzerIO.cpp
index 4bb2df5..abc444a 100644
--- a/llvm/lib/Fuzzer/FuzzerIO.cpp
+++ b/llvm/lib/Fuzzer/FuzzerIO.cpp
@@ -15,6 +15,7 @@
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>
+#include <cstdarg>
 #include <cstdio>
 
 namespace fuzzer {