Add fmt_helper.h include to includes.h and os-inl.h

include/spdlog/details/os-inl.h

@@ -33,6 +33,7 @@
 
 #    if defined(SPDLOG_WCHAR_TO_UTF8_SUPPORT) || defined(SPDLOG_WCHAR_FILENAMES)
 #        include <limits>
+#        include <spdlog/details/fmt_helper.h>
 #    endif
 
 #    include <direct.h> // for _mkdir/_wmkdir

tests/includes.h

@@ -17,6 +17,7 @@
 
 #include "spdlog/spdlog.h"
 #include "spdlog/async.h"
+#include "spdlog/details/fmt_helper.h"
 #include "spdlog/sinks/basic_file_sink.h"
 #include "spdlog/sinks/daily_file_sink.h"
 #include "spdlog/sinks/null_sink.h"

tests/test_fmt_helper.cpp

@@ -1,6 +1,5 @@
 
 #include "includes.h"
-#include "spdlog/details/fmt_helper.h"
 
 using spdlog::memory_buf_t;
 using spdlog::details::fmt_helper::to_string_view;