linux-headers: bump 3.18.x series

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/linux-headers/Config.in.host

@@ -198,7 +198,7 @@ config BR2_DEFAULT_KERNEL_HEADERS
 	default "3.14.52"	if BR2_KERNEL_HEADERS_3_14
 	default "3.16.7"	if BR2_KERNEL_HEADERS_3_16
 	default "3.17.8"	if BR2_KERNEL_HEADERS_3_17
-	default "3.18.20"	if BR2_KERNEL_HEADERS_3_18
+	default "3.18.21"	if BR2_KERNEL_HEADERS_3_18
 	default "3.19.8"	if BR2_KERNEL_HEADERS_3_19
 	default "4.0.9"		if BR2_KERNEL_HEADERS_4_0
 	default "4.1.7"		if BR2_KERNEL_HEADERS_4_1