Update ExtendedBG.c

[sgsilva]

i updated my Herc and when i was compile i got error

ExtendedBG.c: In function ‘area_flooritem’: ExtendedBG.c:2793:6: error: ‘struct map_interface’ has no member named ‘search_freecell’ map->search_freecell(NULL, m, &x, &y, range, range, 1); ^

to fix is olny change it

- map->search_freecell(NULL, m, &x, &y, range, range, 1);

for it

+ map->search_free_cell(NULL, m, &x, &y, range, range, 1);

i was try to make new Pull request, but i can't.