powerpc: Fix fallout from system.h split up

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
diff --git a/arch/powerpc/kernel/kgdb.c b/arch/powerpc/kernel/kgdb.c
index 76a6e40..782bd0a 100644
--- a/arch/powerpc/kernel/kgdb.c
+++ b/arch/powerpc/kernel/kgdb.c
@@ -24,6 +24,7 @@
 #include <asm/current.h>
 #include <asm/processor.h>
 #include <asm/machdep.h>
+#include <asm/debug.h>
 
 /*
  * This table contains the mapping between PowerPC hardware trap types, and