parted: fix ptest RRECOMMENDS

The recipe was using =, which replaces the default RRECOMMENDS from
ptest.bbclass.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>