qt: fix build some more when QPA is not available

Compiler would complain about include directory that didn't
exist because QPA_INCLUDE_PATH gets subst-ed regardless
(and if it didn't we'd have just an empty -I argument).

https://bugzilla.gnome.org/show_bug.cgi?id=767553

configure.ac

@@ -2875,7 +2875,7 @@ AG_GST_CHECK_FEATURE(QT, [Qt elements], qt, [
         QT_VERSION="`$PKG_CONFIG --modversion Qt5Core`"
         QPA_INCLUDE_PATH=`$PKG_CONFIG --variable=includedir Qt5Core`/QtGui/${QT_VERSION}/QtGui
         AS_IF([test -f "$QPA_INCLUDE_PATH/qpa/qplatformnativeinterface.h"], [
-          AC_SUBST(QPA_INCLUDE_PATH)
+          QT_CFLAGS="$QT_CFLAGS -I$QPA_INCLUDE_PATH"
           HAVE_QT_QPA_HEADER="yes"
         ], [AC_MSG_NOTICE([Cannot find QPA])])
         if test "x$GST_GL_HAVE_WINDOW_X11" = "x1" -a "x$GST_GL_HAVE_PLATFORM_GLX" = "x1"; then

ext/qt/Makefile.am

@@ -26,7 +26,6 @@ libgstqtsink_la_SOURCES = \
 libgstqtsink_la_CXXFLAGS = \
 	-I$(top_srcdir)/gst-libs \
 	-I$(top_builddir)/gst-libs \
-	-I$(QPA_INCLUDE_PATH) \
 	$(GST_CXXFLAGS) \
 	$(GST_BASE_CFLAGS) \
 	$(GST_PLUGINS_BASE_CFLAGS) \