Do not set USE_FFI_CLOSURE_ALLOC=1 in fiddle on OpenBSD

On OpenBSD, USE_FFI_CLOSURE_ALLOC was always set to 0 previously. In
633a1f15d8, the code was modified in a
way that it ended up being set to 1 on OpenBSD.  However, that results
in SIGABRT when running make test-all, inside ffi_closure_free.
Setting USE_FFI_CLOSURE_ALLOC back to 0 fixes the issue.

ext/fiddle/closure.c

@@ -13,6 +13,10 @@ typedef struct {
     ffi_type **argv;
 } fiddle_closure;
 
+#if defined(__OpenBSD__)
+# define USE_FFI_CLOSURE_ALLOC 0
+#endif
+
 #if defined(USE_FFI_CLOSURE_ALLOC)
 #elif !defined(HAVE_FFI_CLOSURE_ALLOC)
 # define USE_FFI_CLOSURE_ALLOC 0