[POWERPC] Remove duplicate #include

Remove duplicate #include of <asm/prom.h> in
arch/powerpc/kernel/btext.c.

Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/btext.c

@@ -16,7 +16,6 @@
 #include <asm/mmu.h>
 #include <asm/pgtable.h>
 #include <asm/io.h>
-#include <asm/prom.h>
 #include <asm/processor.h>
 #include <asm/udbg.h>