systemd: needs kernel headers >= 3.0

Systemd requires at least Linux headers 3.0 because it uses:

- setns(2) syscall, introduced in 3.0.
- IFLA_GROUP from if_link.h, introduced in 2.6.39.

Fixes http://autobuild.buildroot.net/results/098/09866da023d527b3c228a98f1fde104e780e1883/

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

system/Config.in

@@ -96,10 +96,12 @@ config BR2_INIT_SYSTEMD
 	depends on BR2_TOOLCHAIN_HAS_SSP
 	depends on BR2_USE_MMU
 	depends on !BR2_PREFER_STATIC_LIB
+	depends on BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_0
 	select BR2_PACKAGE_SYSTEMD
 
-comment 'systemd needs an (e)glibc toolchain'
-	depends on !BR2_TOOLCHAIN_USES_GLIBC
+comment 'systemd needs an (e)glibc toolchain, headers >= 3.0'
+	depends on !(BR2_TOOLCHAIN_USES_GLIBC \
+		&& BR2_TOOLCHAIN_HEADERS_AT_LEAST_3_0)
 
 config BR2_INIT_NONE
 	bool "None"