[nrf noup] tests: secure_storage: fix test w/ ZMS backend on 54L15

noup because it's about partition manager.

Fix the build of secure_storage.psa.its.secure_storage.store.zms on
nrf54l15dk/nrf54l15/cpuapp by disabling partition manager, which is
incompatible with the ZMS implementation of the ITS store module.

Signed-off-by: Tomi Fontanilles <[email protected]>

Author: tomi-font

tests/subsys/secure_storage/psa/its/sysbuild.conf

@@ -0,0 +1 @@
+SB_CONFIG_PARTITION_MANAGER=n