ti-gfx.mk: correct typo

There is a simple typo in package/ti-gfx/ti-gfx.mk, which the
following patch corrects.

Signed-off-by: Martin Creutziger <buildroot@martin.creutziger.net>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>