powerpc: Define a _sdata symbol

This is needed by arch/powerpc/mm/mem.c now.

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/arch/powerpc/kernel/vmlinux.lds.S b/arch/powerpc/kernel/vmlinux.lds.S
index 0587d9c..d0239bf 100644
--- a/arch/powerpc/kernel/vmlinux.lds.S
+++ b/arch/powerpc/kernel/vmlinux.lds.S
@@ -107,6 +107,7 @@
 #ifdef CONFIG_PPC32
   /* Read-write section, merged into data segment: */
   . = ALIGN(4096);
+  _sdata = .;
   .data    :
   {
     *(.data)