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>