Fix missing semicolon in trace_dbuf.h (openzfs#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 <[email protected]>
Reviewed-by: Alexander Motin <[email protected]>
Reviewed-by: Tony Hutter <[email protected]>

include/os/linux/zfs/sys/trace_dbuf.h

@@ -80,7 +80,7 @@
 		snprintf(__get_str(msg), TRACE_DBUF_MSG_MAX,		\
 		    DBUF_TP_PRINTK_FMT, DBUF_TP_PRINTK_ARGS);		\
 	} else {							\
-		__assign_str(os_spa, "NULL")				\
+		__assign_str(os_spa, "NULL");				\
 		__entry->ds_object = 0;					\
 		__entry->db_object = 0;					\
 		__entry->db_level  = 0;					\