test: posix: timers: Fix unused function warning

Building with clang warns:

tests/posix/timers/src/clock.c:37:20: error: unused function 'tv_to_ts'
[-Werror,-Wunused-function]
static inline void tv_to_ts(const struct timeval *tv, struct timespec *ts)
                   ^
tests/posix/timers/src/clock.c:51:16: error: unused function 'tp_eq'
[-Werror,-Wunused-function]
_decl_op(bool, tp_eq, ==);     /* a == b */
               ^
tests/posix/timers/src/clock.c:52:16: error: unused function 'tp_lt'
[-Werror,-Wunused-function]
_decl_op(bool, tp_lt, <);      /* a < b */
               ^
tests/posix/timers/src/clock.c:53:16: error: unused function 'tp_gt'
[-Werror,-Wunused-function]
_decl_op(bool, tp_gt, >);      /* a > b */
               ^
tests/posix/timers/src/clock.c:54:16: error: unused function 'tp_le'
[-Werror,-Wunused-function]
_decl_op(bool, tp_le, <=);     /* a <= b */

tests/posix/timers/src/clock.c:59:20: error: unused function
'tp_diff_in_range_ns' [-Werror,-Wunused-function]
static inline bool tp_diff_in_range_ns(const struct timespec *a,
                   ^                   const struct timespec *b,

tests/posix/timers/src/clock.c:49:20: error: unused function
'tp_diff_in_range_ns' [-Werror,-Wunused-function]
static inline bool tp_diff_in_range_ns(const struct timespec *a,
                   ^                   const struct timespec *b,

Signed-off-by: Tom Hughes <tomhughes@chromium.org>