rs: Fix RS_VERSION define

Commit 51dc7a27a8749d4ad43169bf39d4378529e132cd broke the RS_VERSION
define by setting LOCAL_CFLAGS := something after the addition of
LOCAL_CFLAGS += -DRS_VERSION=...

Make sure the := definition occurs before the += amendment.

Change-Id: I320ae9215f3ef2982c707a6ecaf5a14880ffa036
Signed-off-by: Bernhard Rosenkraenzer <Bernhard.Rosenkranzer@linaro.org>
properly fix rs_version define