powerpc: add context.vdso_base for 32-bit too

This adds a vdso_base element to the mm_context_t for 32-bit compiles
(both for ARCH=powerpc and ARCH=ppc).  This fixes the compile errors
that have been reported in arch/powerpc/kernel/signal_32.c.

Signed-off-by: Paul Mackerras <paulus@samba.org>
diff --git a/include/asm-ppc/mmu.h b/include/asm-ppc/mmu.h
index 9205db4..80ae604 100644
--- a/include/asm-ppc/mmu.h
+++ b/include/asm-ppc/mmu.h
@@ -24,8 +24,10 @@
 #define PHYS_FMT	"%16Lx"
 #endif
 
-/* Default "unsigned long" context */
-typedef unsigned long mm_context_t;
+typedef struct {
+	unsigned long id;
+	unsigned long vdso_base;
+} mm_context_t;
 
 /* Hardware Page Table Entry */
 typedef struct _PTE {