popt: fix override syntax in RDEPENDS

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

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>
(cherry picked from commit 37a47bfc)
Signed-off-by: Steve Sakoman <steve@sakoman.com>