tests: drivers: adc: increase RODATA region size

Increase the rodata section for build_all/adc to
successfully build on the platform atsame54_xpro.

Signed-off-by: Benedikt Schmidt <benedikt.schmidt@embedded-solutions.at>

tests/drivers/build_all/adc/prj.conf

@@ -3,3 +3,4 @@ CONFIG_TEST_USERSPACE=y
 CONFIG_ADC=y
 CONFIG_MAX_THREAD_BYTES=4
 CONFIG_ADC_INIT_PRIORITY=80
+CONFIG_KOBJECT_RODATA_AREA_EXTRA_BYTES=256