QFlags: Mark the mixed enum operator declared in macro maybe_unused

Because, when used in a .cpp file, the compiler can verify that
it is. With warnings-are-errors this can break compilation.

Pick-to: 6.4 6.2
Change-Id: I2c403d914fca24158dba93f618476521cbcfe39f
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/global/qflags.h

@@ -218,6 +218,7 @@ QT_DECLARE_TYPESAFE_OPERATORS_FOR_FLAGS_ENUM(Flags)
 #if __cplusplus > 201702L // assume compilers don't warn if in C++17 mode
   // in C++20 mode, provide user-defined operators to override the deprecated operations:
 # define Q_DECLARE_MIXED_ENUM_OPERATOR(op, Ret, LHS, RHS) \
+    [[maybe_unused]] \
     constexpr inline Ret operator op (LHS lhs, RHS rhs) noexcept \
     { return static_cast<Ret>(qToUnderlying(lhs) op qToUnderlying(rhs)); } \
     /* end */