rtla: Use str_has_prefix() for option prefix check

rtla: Use str_has_prefix() for option prefix check

The argument parsing code in timerlat_main() and osnoise_main() uses
strncmp() with a length of 1 to check if the first argument starts
with a dash, indicating an option flag was passed.

Replace this pattern with str_has_prefix() for consistency with the
rest of the codebase. While character comparison would be slightly
more efficient, using str_has_prefix() provides better readability
and maintains a uniform coding style throughout the rtla tool.

Signed-off-by: Wander Lairson Costa <wander@redhat.com>
Link: https://lore.kernel.org/r/20260309195040.1019085-14-wander@redhat.com
Signed-off-by: Tomas Glozar <tglozar@redhat.com>