libxfs: fix build warnings

libxfs: fix build warnings

gcc 14.2 with all the warnings turn on complains about missing
prototypes for these two functions:

 util.c:147:1: error: no previous prototype for 'current_fixed_time' [-Werror=missing-prototypes]
   147 | current_fixed_time(
       | ^~~~~~~~~~~~~~~~~~
 util.c:590:1: error: no previous prototype for 'get_deterministic_seed' [-Werror=missing-prototypes]
   590 | get_deterministic_seed(
       | ^~~~~~~~~~~~~~~~~~~~~~

Since they're not used outside of util.c, just make them static.

Fixes: 4a54700b4385bb ("libxfs: support reproducible filesystems using deterministic time/seed")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>