add missing header

Author: uzairnawaz

libc/test/src/__support/wchar/string_converter_test.cpp

@@ -8,6 +8,7 @@
 
 // TODO: add support for 16-bit widechars to StringConverter to remove this
 // macro
+#include "src/__support/macros/properties/os.h"
 #ifndef LIBC_TARGET_OS_IS_WINDOWS
 
 #include "hdr/errno_macros.h"
@@ -357,4 +358,4 @@ TEST(LlvmLibcStringConverterTest, DestLimitUTF32To8) {
   ASSERT_EQ(static_cast<int>(sc.getSourceIndex()), 1);
 }
 
-#endif
+#endif