predator-regre/test-0095.c: fix undefined behavior

The test started to hang with gcc-15 because the union was not fully
initialized.  Unlike with struct, there is no guarantee that the whole
union object will be initialized if a single member initializer is
specified.  So the test has been working only by accident.  This commit
makes the union static to enforce initialization.

Closes: #105

tests/predator-regre/test-0095.c

@@ -27,7 +27,7 @@ union data {
 
 int main()
 {
-    union data d = {
+    static union data d = {
         .number = 0L
     };
     __VERIFIER_plot(NULL);