Fix missing semicolon in trace_dbuf.h (#16281)

On fedora 40, on the 6.9.4 kernel (in updates-testing), assign_str
expands to a "do {<stuff> } while(0)" loop. Without this semicolon,
the while(0) is unterminated, causing a cascade of useless errors.
With this semicolon, it compiles fine. It also compiles fine on 6.8.11
(the previous kernel). I have not tested earlier kernels than that, but
at worst it should add a pointless semicolon.

All other instances in the source tree are already terminated with
semicolons.

Signed-off-by: Daniel Berlin <dberlin@dberlin.org>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>