pgsql: 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

Branch
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REL_13_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/065683bbdbab7fe51c59a479f4136328577bbecd

Modified Files
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contrib/pg_trgm/trgm_gist.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)