Work with srcdir != builddir.

Work with srcdir != builddir.
* demos/testanimation.c: Don't include "gdk-pixbuf.h".  Use
<gtk/gdk-pixbuf-loader.h>, not "gtk-pixbuf/gdk-pixbuf-loader.h".
* demos/testpixbuf.c: Likewise.
* demos/testpixbuf-scale.c: Don't include "gdk-pixbuf.h".
* demos/testpixbuf-drawable.c: Don't include "gdk-pixbuf.h".  Use
<gdk/x11/gdkx.h>.
* demos/pixbuf-demo.c: Use <gtk/gtk.h> not "gtk.h".  Don't include
"gdk-pixbuf.h".
* demos/Makefile.am (INCLUDES): Remove unneeded directories.