linux-headers: bump 3.18.x version to 3.18.57

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/linux-headers/Config.in.host

@@ -218,7 +218,7 @@ config BR2_DEFAULT_KERNEL_HEADERS
 	default "3.4.113"	if BR2_KERNEL_HEADERS_3_4
 	default "3.10.105"	if BR2_KERNEL_HEADERS_3_10
 	default "3.12.74"	if BR2_KERNEL_HEADERS_3_12
-	default "3.18.55"	if BR2_KERNEL_HEADERS_3_18
+	default "3.18.57"	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 "4.1.40"	if BR2_KERNEL_HEADERS_4_1