[NFC][threads] Ignore type-ssa-shared.wast in fuzzer (#6754)

The fuzzer does not yet properly handle initial contents containing
shared types.

Author: tlively

scripts/fuzz_opt.py

@@ -357,6 +357,7 @@ def is_git_repo():
     'shared-struct.wast',
     'shared-array.wast',
     'shared-i31.wast',
+    'type-ssa-shared.wast',
 ]