remove duplicated include

[Huang Weiyi]

Removed duplicated include file <asm/prom.h> in powerpc/kernel/btext.c.

Signed-off-by: Huang Weiyi <weiyi.huang at gmail.com>

--- a/powerpc/kernel/btext.c    2008-05-08 21:20:20.000000000 +0800
+++ b/powerpc/kernel/btext.c    2008-05-08 21:20:38.000000000 +0800
@@ -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>