libgc: Fix crasher with some GCC configurations
libgc: Fix crasher with some GCC configurations

const variables are not supposed to be writable, and trying to
initialise the various "const" blocks caused SEGVs. Remove the const
modifier as it's incorrect.

The valgrind output for this crash is clear, "Bad permissions":
==11056== Process terminating with default action of signal 11 (SIGSEGV): dumping core
==11056==  Bad permissions for mapped region at address 0x28C91360
==11056==    at 0x28C8D0AB: init_blocks (gc.c:926)
==11056==    by 0x28C8E46B: gc_character_iter_init_for_related (gc.c:938)
==11056==    by 0x28C8E46B: gc_search_context_search (gc.c:1124)

GNOME/gnome-characters,GNOME/gnome-characters,GNOME/gnome-characters,GNOME/gnome-characters,GNOME/gnome-characters