[libFuzzer] Clean up headers and file formatting of LibFuzzer files.

Reorganize #includes to follow LLVM Coding Standards.
Include some missing headers. Required to use `Printf()`.

Aside from that, this patch contains no functional change.
It is purely a re-organization.

Differential Revision: https://reviews.llvm.org/D27363

llvm-svn: 289560
diff --git a/llvm/lib/Fuzzer/FuzzerIOPosix.cpp b/llvm/lib/Fuzzer/FuzzerIOPosix.cpp
index a4290fa..53a58f9 100644
--- a/llvm/lib/Fuzzer/FuzzerIOPosix.cpp
+++ b/llvm/lib/Fuzzer/FuzzerIOPosix.cpp
@@ -8,9 +8,9 @@
 //===----------------------------------------------------------------------===//
 // IO functions implementation using Posix API.
 //===----------------------------------------------------------------------===//
-
 #include "FuzzerDefs.h"
 #if LIBFUZZER_POSIX
+
 #include "FuzzerExtFunctions.h"
 #include "FuzzerIO.h"
 #include <cstdarg>
@@ -84,4 +84,5 @@
 }
 
 }  // namespace fuzzer
+
 #endif // LIBFUZZER_POSIX