[AVR32] Fix typo in include/asm-avr32/Kbuild

It's header-y, not headers-y.

Signed-off-by: Haavard Skinnemoen <hskinnemoen@atmel.com>
diff --git a/include/asm-avr32/Kbuild b/include/asm-avr32/Kbuild
index 8770e73..3136628 100644
--- a/include/asm-avr32/Kbuild
+++ b/include/asm-avr32/Kbuild
@@ -1,3 +1,3 @@
 include include/asm-generic/Kbuild.asm
 
-headers-y	+= cachectl.h
+header-y	+= cachectl.h