QNetworkRequest: export the static constexpr (inline) member variables

All that is required is that we write the declaration as if they weren't
inline, like we used to for C++98 non-constexpr static members. Amends
090991123d.

This is required with MinGW and happens with both GCC and Clang: when a
static constexpr variable found in an exported class is used in a
context that requires getting its address, the compiler emit a DLL-
importing statement. For example:

 void f(const std::chrono::milliseconds &ms);
 void f()
 {
    f(QNetworkRequest::DefaultTransferTimeout);
 }

Emits with GCC:
 _Z1fv:
        movq    __imp__ZN15QNetworkRequest22DefaultTransferTimeoutE(%rip), %rcx
        jmp     _Z1fRKNSt6chrono8durationIxSt5ratioILx1ELx1000EEEE

And Clang:
 _Z1fv:                                  # @_Z1fv
 # %bb.0:
        movq    __imp__ZN15QNetworkRequest22DefaultTransferTimeoutE(%rip), %rcx
        jmp     _Z1fRKNSt6chrono8durationIxSt5ratioILx1ELx1000EEEE # TAILCALL

Pick-to: 6.7
Fixes: QTBUG-121135
Change-Id: I76ffba14ece04f24b43efffd17aae1a98071c570
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>
Reviewed-by: André Klitzing <aklitzing@gmail.com>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

src/network/access/qnetworkrequest.cpp

@@ -30,6 +30,8 @@ QT_BEGIN_NAMESPACE
 using namespace Qt::StringLiterals;
 using namespace std::chrono_literals;
 
+constexpr std::chrono::milliseconds QNetworkRequest::DefaultTransferTimeout;
+
 QT_IMPL_METATYPE_EXTERN(QNetworkRequest)
 QT_IMPL_METATYPE_EXTERN_TAGGED(QNetworkRequest::RedirectPolicy, QNetworkRequest__RedirectPolicy)