Fix build with qmake

Since the direct .moc inclusion was added for cmake, we need to make the
_p.h visible to qmake via HEADERS so that qmake can also see this.

Change-Id: I042544856fe28e46b978d3b5938fbff8614367be
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>

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

@@ -9,5 +9,6 @@ win32: QMAKE_USE += ws2_32
 unix:qtConfig(reduce_exports) {
     SOURCES += $$QNETWORK_SRC/socket/qnativesocketengine_unix.cpp
     SOURCES += $$QNETWORK_SRC/socket/qnativesocketengine.cpp
+    HEADERS += $$QNETWORK_SRC/socket/qnativesocketengine_p.h
     SOURCES += $$QNETWORK_SRC/socket/qabstractsocketengine.cpp
 }