propvarutil.h: Add extern "C" if __cplusplus is defined.

Wine-Bug: https://bugs.winehq.org/show_bug.cgi?id=46939
Signed-off-by: Austin English <austinenglish@gmail.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>
(cherry picked from commit da998cb206)
Signed-off-by: Michael Stefaniuc <mstefani@winehq.org>

include/propvarutil.h

@@ -22,6 +22,10 @@
 #include <shtypes.h>
 #include <shlwapi.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif
+
 enum tagPROPVAR_CHANGE_FLAGS
 {
     PVCHF_DEFAULT           = 0x00000000,
@@ -117,7 +121,12 @@ inline HRESULT InitPropVariantFromInt64(LONGLONG llVal, PROPVARIANT *ppropvar)
     return S_OK;
 }
 
-#endif
+#endif /* NO_PROPVAR_INLINES */
+#endif /* __cplusplus */
+
+
+#ifdef __cplusplus
+}
 #endif
 
 #endif /* __WINE_PROPVARUTIL_H */