meson.build: Update default status for jlink_spi

In commit 07dd1219ff15 (Makefile: Enable jlink_spi by default) we
forgot to update the `meson.build` file. We actually never needed
the `'default' : false` because our Meson setup would disable it
automatically if the library is missing.

Change-Id: I9c68d00a7cbf3c8be50cd1e58e54262bf14bd779
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/flashrom-stable/+/73855
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
1 file changed