[Debuginfod] Restore a couple of includes

This patch adds back a couple of includes to HTTPClient.cpp.

A breakage has been reported in:

#142852

Author: kazutakahirata

llvm/lib/Debuginfod/HTTPClient.cpp

@@ -15,7 +15,9 @@
 #include "llvm/Debuginfod/HTTPClient.h"
 #include "llvm/ADT/APInt.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Errc.h"
 #include "llvm/Support/Error.h"
+#include "llvm/Support/MemoryBuffer.h"
 #ifdef LLVM_ENABLE_CURL
 #include <curl/curl.h>
 #endif