clutter-gst: Fix broken build for images depending on gobject-introspection-native...

clutter-gst: Fix broken build for images depending on gobject-introspection-native (e.g. libwnck_2.30.5 / libgee_0.6.0)

* Error situation:
* 1. configure detects gobject-introspection:
*
configure:12133: checking for gobject-introspection
configure:12164: $PKG_CONFIG --exists --print-errors "gobject-introspection-1.0 >= 0.6.8"
*
* 2. compile fails:
*
  CCLD   libclutter-gst-1.0.la
  GEN    ClutterGst-1.0.gir
Couldn't find include 'Clutter-1.0.gir' (search path: ['.', 'path=.', '/home/Superandi/tmp/sysroots/armv7a-angstrom-linux-gnueabi/usr/share/gir-1.0', '/home/Superandi/tmp/sysroots/i686-linux/usr/share/gir-1.0', '/usr/share/gir-1.0', '/home/Superandi/tmp/sysroots/i686-linux/usr/share/gir-1.0'])
make[3]: *** [ClutterGst-1.0.gir] Error 1
*
* After fix configure disables gobject-introspection
*
configure:12133: checking for gobject-introspection
configure:12178: result: no (disabled, use --enable-introspection to enable)

Signed-off-by: Andreas Mueller <schnitzeltony@gmx.de>
Signed-off-by: Khem Raj <raj.khem@gmail.com>