powerpc: Fix typo "CONFIG_ICSWX_PID"

Untested. As this typo was introduced in v3.3, with commit
9d670280 ("powerpc: Split ICSWX ACOP and
PID processing"), which actually added PPC_ICSWX_PID, this surely needs
testing.
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>

arch/powerpc/mm/icswx.c

@@ -67,7 +67,7 @@
 
 void switch_cop(struct mm_struct *next)
 {
-#ifdef CONFIG_ICSWX_PID
+#ifdef CONFIG_PPC_ICSWX_PID
 	mtspr(SPRN_PID, next->context.cop_pid);
 #endif
 	mtspr(SPRN_ACOP, next->context.acop);