git: fix override syntax in RDEPENDS

RDEPENDS_${PN}-tk -> RDEPENDS:${PN}-tk

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>