[Debuginfod] Restore an include

A breakage has been reported in:

#142852

Author: kazutakahirata

llvm/lib/Debuginfod/HTTPServer.cpp

@@ -16,6 +16,7 @@
 #include "llvm/Debuginfod/HTTPServer.h"
 #include "llvm/ADT/StringExtras.h"
 #include "llvm/ADT/StringRef.h"
+#include "llvm/Support/Errc.h"
 #include "llvm/Support/Error.h"
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/MemoryBuffer.h"