pixman: add submodule

Add pixman submodule as fallback for old distros.
Picking version 0.18.4.  This is shipped by rhel6
and also the minimum version needed by spice so this
should serve well as baseline.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

.gitmodules

@@ -19,3 +19,6 @@
 [submodule "roms/sgabios"]
 	path = roms/sgabios
 	url = git://git.qemu.org/sgabios.git
+[submodule "pixman"]
+	path = pixman
+	url = git://anongit.freedesktop.org/pixman