Fix GUC boot values

[df7cb]

Fixes assertion failures on cassert-enabled PG16.

LOG:  GUC (PGC_REAL) pg_similarity.euclidean_threshold, boot_val=0.7, C-var=0.7
STATEMENT:  CREATE EXTENSION pg_similarity;
TRAP: failed Assert("check_GUC_init(variable)"), File: "./build/../src/backend/utils/misc/guc.c", Line: 4764, PID: 779990

(Fix developed against 1.0 - please do tag a new version, 1.0 is quite old!)

Close: #39