system/Config.in: update systemd dependencies

Commit eb49354eb34842f88218fcf9ef60576b85653e38 updated systemd's
minimal kernel headers version to 3.13, but forgot to update the
dependencies of BR2_INIT_SYSTEMD.

Update BR2_INIT_SYSTEMD kernel headers dependency to 3.13.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

system/Config.in

@@ -125,7 +125,7 @@ config BR2_INIT_SYSTEMD
 	depends on BR2_TOOLCHAIN_USES_GLIBC
 	depends on BR2_TOOLCHAIN_HAS_SSP
 	depends on BR2_TOOLCHAIN_HAS_THREADS
-	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_10
+	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_13
 	depends on BR2_TOOLCHAIN_GCC_AT_LEAST_5
 	depends on BR2_HOST_GCC_AT_LEAST_5
 	select BR2_ROOTFS_MERGED_USR