[llvm-symbolizer] Add missing include for config.h

[akhuang]

Summary

The cmake variable LLVM_ENABLE_DIA_SDK was being used here but
was undefined because config.h wasn't included.

[MaskRay]

LGTM.

llvm/tools/llvm-symbolizer/llvm-symbolizer.cpp

 // file, line number, column number). It is targeted for compiler-rt tools
 // (especially AddressSanitizer and ThreadSanitizer) that can use it
 // to symbolize stack traces in their error reports.
 //
 //===----------------------------------------------------------------------===//
 
 #include "Opts.inc"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Config/config.h"
 #include "llvm/DebugInfo/Symbolize/DIPrinter.h"
 #include "llvm/DebugInfo/Symbolize/Symbolize.h"
 #include "llvm/Option/Arg.h"
 #include "llvm/Option/ArgList.h"
 #include "llvm/Option/Option.h"
 #include "llvm/Support/COM.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Debug.h"