testsuite: Fix gcc.dg/vect/pr99102.c

pr99102.c needs to override the default options to exercise the original
problem, but that means that it also needs to respecify the dump flags.

gcc/testsuite/
	* gcc.dg/vect/pr99102.c: Add -fdump-tree-vect-details.

gcc/testsuite/gcc.dg/vect/pr99102.c

-/* { dg-options "-O2 -ftree-vectorize" } */
+/* { dg-options "-O2 -ftree-vectorize -fdump-tree-vect-details" } */
 /* { dg-additional-options "-msve-vector-bits=256" { target aarch64_sve256_hw } } */
 long a[44];
 short d, e = -7;