testsuite: Require vectors of doubles for pr97428.c

The pr97428.c test assumes support for vectors of doubles, but some
targets only support vectors of floats, causing this test to fail with
such targets.  Limit this test to targets that support vectors of
doubles then.

	gcc/testsuite/
	* gcc.dg/vect/pr97428.c: Limit to `vect_double' targets.

gcc/testsuite/gcc.dg/vect/pr97428.c

 /* { dg-do compile } */
+/* { dg-require-effective-target vect_double } */
 
 typedef struct { double re, im; } dcmlx_t;
 typedef struct { double re[4], im[4]; } dcmlx4_t;