misc: Turn sstk into a compat symbol

It is not implemented anywhere.  There is an osf_sstk system call on
alpha, but it is not used to implement sstk, and the system call
is not implemented on Linux, either.

misc/sstk.c

@@ -16,17 +16,14 @@
    <https://www.gnu.org/licenses/>.  */
 
 #include <errno.h>
+#include <shlib-compat.h>
 
-/* Increase the size of the stack by INCREMENT,
-   and return the address of the bottom of the stack.  */
-
-void *sstk (int increment) __THROW;
-
+#if SHLIB_COMPAT (libc, GLIBC_2_0, GLIBC_2_32)
 void *
 sstk (int increment)
 {
   __set_errno (ENOSYS);
   return (void *) -1;
 }
-
-stub_warning (sstk)
+compat_symbol (libc, sstk, sstk, GLIBC_2_0);
+#endif