librem5-devkit: Enable Rocktech panel

The Kconfig variable name changed upstream

Signed-off-by: Guido Günther <guido.gunther@puri.sm>
3 jobs for devkit-panel-upstream in 53 minutes and 6 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Build
passed build:aarch64:Image #11980
librem5

00:32:45

passed build:aarch64:deb #11969
librem5

00:20:20

failed build:aarch64:Image #11968
librem5

00:01:58