Non-functional change: Fix a compilation warning introduced in r4144:

gcc -DHAVE_CONFIG_H -I. -I../../include -I../../include -I../../include -I../../regex -I../../storage/innobase/include -I../../sql -I.   -Werror 	 -Wall -g   -MT libinnobase_a-sync0arr.o -MD -MP -MF .deps/libinnobase_a-sync0arr.Tpo -c -o libinnobase_a-sync0arr.o `test -f 'sync/sync0arr.c' || echo './'`sync/sync0arr.c
cc1: warnings being treated as errors
sync/sync0arr.c: In function 'sync_array_object_signalled':
sync/sync0arr.c:869: warning: pointer targets in passing argument 1 of 'os_atomic_increment' differ in signedness