powerpc: fix system.h fallout in sysdev/scom.c [chroma_defconfig]

The following shows up in chroma_defconfig:

 CC      arch/powerpc/sysdev/scom.o
arch/powerpc/sysdev/scom.c: In function 'scom_debug_init':
arch/powerpc/sysdev/scom.c:182:36: error: 'powerpc_debugfs_root' undeclared (first use in this function)
arch/powerpc/sysdev/scom.c:182:36: note: each undeclared identifier is reported only once for each function it appears in
make[2]: *** [arch/powerpc/sysdev/scom.o] Error 1
make[1]: *** [arch/powerpc/sysdev/scom.o] Error 2

A bisect leads to commit 9ffc93f2

    "Remove all #inclusions of asm/system.h"

Add the debug header which contains powerpc_debugfs_root.
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>

arch/powerpc/sysdev/scom.c

@@ -22,6 +22,7 @@
 #include <linux/debugfs.h>
 #include <linux/slab.h>
 #include <linux/export.h>
+#include <asm/debug.h>
 #include <asm/prom.h>
 #include <asm/scom.h>