Fix qmake builds of socket tests

qabstractsocketengine.cpp was changed to explicitly include its header
moc file, which works with AUTOMOC, but doesn't with qmake.
The header file needs to be explicitly listed in the HEADERS variable.

Amends 2a1fc7b835.

Change-Id: I9d31288d1f1eb2310679a7e685bd7ea458a77920
Reviewed-by: Leander Beernaert <leander.beernaert@qt.io>
Reviewed-by: Qt CMake Build Bot
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>

tests/auto/network/socket/platformsocketengine/platformsocketengine.pri

@@ -11,4 +11,5 @@ unix:qtConfig(reduce_exports) {
     SOURCES += $$QNETWORK_SRC/socket/qnativesocketengine.cpp
     HEADERS += $$QNETWORK_SRC/socket/qnativesocketengine_p.h
     SOURCES += $$QNETWORK_SRC/socket/qabstractsocketengine.cpp
+    HEADERS += $$QNETWORK_SRC/socket/qabstractsocketengine_p.h
 }