Add the gstreamer common submodule.

Signed-off-by: Jacob Stiffler <j-stiffler@ti.com>

diff --git a/common b/common
new file mode 160000 (submodule)
index 0000000..011bcc8
--- /dev/null
+++ b/common
@@ -0,0 +1 @@
+Subproject commit 011bcc8a0fc7f798ee874a7ba899123fb2470e22