Fixed possible dependency problem in the qmake structure

If gstreamer is present, then the gstreamer plugin (and
hence the plugins sub directory) should depend on the
gst tools library.

Change-Id: I6012df02d14731107fc0656089aa27ae61f0ef71
Reviewed-by: Dmytro Poplavskiy <dmytro.poplavskiy@nokia.com>

src/src.pro

@@ -23,7 +23,13 @@ SUBDIRS += \
     src_imports
 
 # Optional bits
-contains(config_test_gstreamer, yes):SUBDIRS += src_qgsttools
+contains(config_test_gstreamer, yes) {
+    SUBDIRS += src_qgsttools
+
+    # If gstreamer is present, then plugins should depend on it
+    src_plugins.depends += src_qgsttools
+}
+
 !isEmpty(QT.widgets.name) {
     SUBDIRS += src_qtmmwidgets