Fix compilation on MSVC

MSVC complains about `(long double){}` so let's rephrase it.

PiperOrigin-RevId: 590297660

Author: majnemer

ml_dtypes/_src/int4_numpy.h

@@ -165,7 +165,8 @@ bool CastToInt4(PyObject* arg, T* output) {
     return floating_conversion(double{});
   }
   if (PyArray_IsScalar(arg, LongDouble)) {
-    return floating_conversion((long double){});
+    using ld = long double;
+    return floating_conversion(ld{});
   }
   return false;
 }