Fix string_view constructor for macOS clang

Some versions of clang 14 (macOS+android) don't implement
`string_view(It first, It last)`, so let's use
`string_view(const CharT*, size_type)` instead.

Also remove unused header leftover from old code that uses std::string.

src/core/gdbstub/hio.h

@@ -4,8 +4,6 @@
 
 #pragma once
 
-#include <string>
-
 #include "common/common_types.h"
 
 namespace GDBStub {