externals: Update httplib

Since the introduction of this library, numerous improvements have been
made. Notably, many of the warnings we would get by simply including the
library header have now been fixed. This makes it much easier to make
conversion warning an error.

src/core/crypto/key_manager.cpp

@@ -21,8 +21,8 @@
 #include "common/common_paths.h"
 #include "common/file_util.h"
 #include "common/hex_util.h"
-#include "common/string_util.h"
 #include "common/logging/log.h"
+#include "common/string_util.h"
 #include "core/core.h"
 #include "core/crypto/aes_util.h"
 #include "core/crypto/key_manager.h"