decompressors: fix typo "POWERPC"

Commit 5dc49c75 ("decompressors: make the default XZ_DEC_* config
match the selected architecture") added

	default y if POWERPC

to lib/xz/Kconfig.  But there is no Kconfig symbol POWERPC.  The most
general Kconfig symbol for the powerpc architecture is PPC.  So let's
use that.
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Cc: Florian Fainelli <florian@openwrt.org>
Cc: Lasse Collin <lasse.collin@tukaani.org>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/xz/Kconfig

@@ -15,7 +15,7 @@ config XZ_DEC_X86
 
 config XZ_DEC_POWERPC
 	bool "PowerPC BCJ filter decoder"
-	default y if POWERPC
+	default y if PPC
 	select XZ_DEC_BCJ
 
 config XZ_DEC_IA64