Under musl a compilation unit cannot include both linux/kernel.h
and sys/sysinfo.h at the same time since that leads to a redefinition
of `struct sysinfo`.

Change the autoconf template to include linux/kernel.h while testing
for sys/sysinfo.h to test for that conflict.

Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>