Add a couple of casts to scan_thread_data () to shut GCC up.

This would result in the following warnings on e.g. PowerPC:

sgen-gc.c: In function 'scan_thread_data':
sgen-gc.c:4045:6: warning: passing argument 1 of 'conservatively_pin_objects_from' from incompatible pointer type [enabled by default]
sgen-gc.c:1467:1: note: expected 'void **' but argument is of type 'void * (*)[32]'
sgen-gc.c:4045:6: warning: passing argument 2 of 'conservatively_pin_objects_from' from incompatible pointer type [enabled by default]
sgen-gc.c:1467:1: note: expected 'void **' but argument is of type 'void * (*)[32]'