BUG/MEDIUM: build: Fix compilation by spelling decl correctly.

Fix build on architectures for which double-width CAS isn't implemented by
spelling __decl_rwlock correctly.
diff --git a/src/fd.c b/src/fd.c
index e790f3e..d026b15 100644
--- a/src/fd.c
+++ b/src/fd.c
@@ -336,7 +336,7 @@
 }
 
 #ifndef HA_HAVE_CAS_DW
-__decl_hathreads(__delc_rwlock(fd_mig_lock));
+__decl_hathreads(__decl_rwlock(fd_mig_lock));
 #endif
 
 /*