[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/FuzzerExtFunctions.h b/llvm/lib/Fuzzer/FuzzerExtFunctions.h
index 2ec86cb..2672a38 100644
--- a/llvm/lib/Fuzzer/FuzzerExtFunctions.h
+++ b/llvm/lib/Fuzzer/FuzzerExtFunctions.h
@@ -8,6 +8,7 @@
 //===----------------------------------------------------------------------===//
 // Defines an interface to (possibly optional) functions.
 //===----------------------------------------------------------------------===//
+
 #ifndef LLVM_FUZZER_EXT_FUNCTIONS_H
 #define LLVM_FUZZER_EXT_FUNCTIONS_H
 
@@ -30,4 +31,5 @@
 #undef EXT_FUNC
 };
 } // namespace fuzzer
+
 #endif