powerpc: Fix build problem with default ppc_md.progress commit

a9c0f41b breaks the build
on some platforms. The extern declaration must be shielded
against assembly.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/setup.h

@@ -3,6 +3,8 @@
 
 #include <asm-generic/setup.h>
 
+#ifndef __ASSEMBLY__
 extern void ppc_printk_progress(char *s, unsigned short hex);
+#endif
 
 #endif	/* _ASM_POWERPC_SETUP_H */