Really really fix use of wxTranslateFromUnicodeFormat() in the unit test.

Don't build this test at all under Unix, where this function doesn't exist.

(Really really) closes #16118.

tests/datetime/datetimetest.cpp

@@ -1449,6 +1449,8 @@ void DateTimeTestCase::TestDateOnly()
     CPPUNIT_ASSERT_EQUAL( wxDateTime::Today(), wxDateTime::Now().GetDateOnly() );
 }
 
+#if defined(__WINDOWS__) || defined(__WXOSX__)
+
 // This function is defined in src/common/intl.cpp and as it is not public we
 // need to declare it here explicitly.
 WXDLLIMPEXP_BASE
@@ -1484,4 +1486,6 @@ void DateTimeTestCase::TestTranslateFromUnicodeFormat()
         wxTranslateFromUnicodeFormat("''H 'o''clock: It''s about time'''"));
 }
 
+#endif // ports having wxTranslateFromUnicodeFormat()
+
 #endif // wxUSE_DATETIME