Don't force -fvisibility=hidden on Windows

Fix #5750

Author: moi15moi

include/pybind11/detail/pybind11_namespace_macros.h

@@ -74,7 +74,7 @@
 // requires forcing hidden visibility on pybind code, so we enforce this by setting the attribute
 // on the main `pybind11` namespace.
 #if !defined(PYBIND11_NAMESPACE)
-#    ifdef __GNUG__
+#    if defined(__GNUG__) && !defined(_WIN32)
 #        define PYBIND11_NAMESPACE pybind11 __attribute__((visibility("hidden")))
 #    else
 #        define PYBIND11_NAMESPACE pybind11