selftests: timers: valid-adjtimex: build fix for newer toolchains

Toolchains with an include file 'sys/timex.h' based on 3.18 will have a
'clock_adjtime' definition added, so it can't be static in the code:

valid-adjtimex.c:43:12: error: static declaration of ‘clock_adjtime’ follows non-static declaration

Fixes: e03a58c3 ("kselftests: timers: Add adjtimex SETOFFSET validity tests")
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: John Stultz <jstultz@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>