tests: logging: log_backend_fs: add missing include

Include explicitly `logging/log_backend.h` header so that non POSIX
boards can build this test. For POSIX boards, this headers was indirectly
included through `ztest.h`->`ztest_assert.h`->`tc_util.h`->`log_ctrl.h`.

Fixes #85441

Signed-off-by: Manuel Argüelles <manuel.arguelles@nxp.com>

Author: manuargue

tests/subsys/logging/log_backend_fs/src/log_fs_test.c

@@ -16,6 +16,7 @@
 #include <zephyr/ztest.h>
 #include <zephyr/fs/fs.h>
 #include <zephyr/fff.h>
+#include <zephyr/logging/log_backend.h>
 
 #define DT_DRV_COMPAT zephyr_fstab_littlefs
 #define TEST_AUTOMOUNT DT_PROP(DT_DRV_INST(0), automount)