Move back FMT_BEGIN_NAMESPACE

Author: localspook

src/os.cc

@@ -59,8 +59,6 @@
 #  endif
 #endif
 
-FMT_BEGIN_NAMESPACE
-
 namespace {
 #ifdef _WIN32
 // Return type of read and write functions.
@@ -79,6 +77,8 @@ inline auto convert_rwcount(size_t count) -> size_t { return count; }
 #endif
 }  // namespace
 
+FMT_BEGIN_NAMESPACE
+
 #ifdef _WIN32
 namespace detail {