Fix name of the macro for getting signature length trgm_gist.c

Fix name of the macro for getting signature length trgm_gist.c

911e702077 has introduced the opclass parameters including signature length
for a set of GiST opclasses.  Due to copy-pasting, macro for getting the
signature length in trgm_gist.c was named LTREE_GET_ASIGLEN().  Fix that by
renaming this macro to just GET_SIGLEN().

Backpatch-through: 13