powerpc/powernv: Add missing include to LPC code

kbuild bot spotted that one:

  arch/powerpc/platforms/powernv/opal-lpc.c: In function 'opal_lpc_init_debugfs':
>> arch/powerpc/platforms/powernv/opal-lpc.c:319:35: error: 'powerpc_debugfs_root' undeclared (first use in this function)
     root = debugfs_create_dir("lpc", powerpc_debugfs_root);
                                      ^
We neet to include the definition explicitely.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/platforms/powernv/opal-lpc.c

@@ -22,6 +22,7 @@
 #include <asm/opal.h>
 #include <asm/prom.h>
 #include <asm/uaccess.h>
+#include <asm/debug.h>
 
 static int opal_lpc_chip_id = -1;