analyzer/strndup-1.c: skip on *-*-vxworks*

Add vxworks to the set of operating systems whose C libraries don't
support strndup.


for  gcc/testsuite/ChangeLog

	* gcc.dg/analyzer/strndup-1.c: Add *-*-vxworks* to no-strndup
	in libc.

gcc/testsuite/gcc.dg/analyzer/strndup-1.c

@@ -1,4 +1,5 @@
-/* { dg-skip-if "no strndup in libc" { *-*-darwin[789]* *-*-darwin10* *-*-mingw* } } */
+/* { dg-skip-if "no strndup in libc" { *-*-darwin[789]* *-*-darwin10* *-*-mingw* *-*-vxworks* } } */
+
 #include <string.h>
 #include <stdlib.h>