qsocketnotifier.cpp: un-break -unity-build

qsocketnotifier.cpp is using a #define to activate a second overload
of QSocketNotifier's activated() signal.

In unity-builds, this overload becomes visible to other TUs and breaks
new-style connect()s¹. In PCH builds, the define comes too late (the
qsocketnotifier.h header was already included by the pch header,
without the define).

Fix by adding qsocketnotifier.cpp to NO_PCH_SOURCES (thereby to
NO_UNITY_BUILD_SOURCES, too).

¹ and the signal's use of QPrivateSignal makes disambiguation by
  qOverload(), or assignment to a function pointer, impossible.

Amends 487dd80bce9c6006f349ccb09222e1c308200f0a(!).

Pick-to: 6.9 6.8 6.5 5.15
Change-Id: I40ca3b90f7ecc3116ae78dc952583efa299bcedb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

src/corelib/CMakeLists.txt

@@ -336,6 +336,7 @@ qt_internal_add_module(Core
     NO_PCH_SOURCES
         compat/removed_api.cpp
         global/qsimd.cpp
+        kernel/qsocketnotifier.cpp # defines BUILDING_QSOCKETNOTIFIER
     DEFINES
         QT_NO_CONTEXTLESS_CONNECT
         QT_NO_FOREACH