tests: fs: fat_fs_api: add native_sim to platform_allow

Native sim was inadvertently forgotten from the platform_allow list with
commit 257e56c.
It should be in there as it is an integration platform.

Signed-off-by: Benjamin Cabé <[email protected]>

Author: kartben

tests/subsys/fs/fat_fs_api/testcase.yaml

@@ -20,6 +20,7 @@ tests:
     filter: dt_compat_enabled("zephyr,sdmmc-disk")
   filesystem.fat.ram.api:
     platform_allow:
+      - native_sim
       - frdm_mcxa156
       - frdm_mcxn236
       - frdm_mcxn947/mcxn947/cpu0