QDBusSignature: de-inline the default ctor

It used to merely default-construct the QString member (which is just
zero-initialization), but since we discovered that we need to set
m_signature to empty instead of null, it's doing too much for inline
code (temporary QString (atomics), compile-time), so de-inline.

Amends ed6d1fa71a, but since this is
adding a new symbol on non-Windows platforms, only pick to unreleased
branches.

Change-Id: I610dad86ac4a080f5a45495ebc9536aa1d0bcbea
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 271876bbcecedccf66179bc46c26e78d04732d6c)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/dbus/qdbusextratypes.cpp

@@ -34,6 +34,10 @@ void QDBusObjectPath::doCheck()
     }
 }
 
+QDBusSignature::QDBusSignature() noexcept
+    : m_signature(QLatin1StringView("")) // mark non-null (empty signatures are valid)
+{}
+
 void QDBusSignature::doCheck()
 {
     if (!QDBusUtil::isValidSignature(m_signature)) {

src/dbus/qdbusextratypes.h

@@ -77,9 +77,7 @@ class Q_DBUS_EXPORT QDBusSignature
 {
     QString m_signature;
 public:
-    QDBusSignature() noexcept
-        : m_signature(QLatin1StringView("")) // mark non-null (empty signatures are valid)
-    {}
+    QDBusSignature() noexcept;
     // compiler-generated copy/move constructor/assignment operators are ok!
     // compiler-generated destructor is ok!