Silence Clang 10 compiler warnings. Fixes pybind#2225.

Author: apollo13

include/pybind11/detail/common.h

@@ -676,7 +676,7 @@ inline void ignore_unused(const int *) { }
 #define PYBIND11_EXPAND_SIDE_EFFECTS(PATTERN) (((PATTERN), void()), ...)
 #else
 using expand_side_effects = bool[];
-#define PYBIND11_EXPAND_SIDE_EFFECTS(PATTERN) pybind11::detail::expand_side_effects{ ((PATTERN), void(), false)..., false }
+#define PYBIND11_EXPAND_SIDE_EFFECTS(PATTERN) (void)pybind11::detail::expand_side_effects{ ((PATTERN), void(), false)..., false }
 #endif
 
 PYBIND11_NAMESPACE_END(detail)