testsuite: PPC vector double requires VSX

pr101168.C uses __vector double, which requires VSX, not just Altivec.
This patch invokes the testcase with the -mvsx option.

	* g++.target/powerpc/pr101168.C: Require VSX.

gcc/testsuite/g++.target/powerpc/pr101168.C

@@ -1,6 +1,6 @@
 /* { dg-do compile } */
-/* { dg-require-effective-target powerpc_altivec_ok } */
-/* { dg-options "-maltivec" } */
+/* { dg-require-effective-target powerpc_vsx_ok } */
+/* { dg-options "-mvsx" } */
 
 using vdbl =  __vector double;
 #define BREAK 1