Let configure include `sys/if_tun.h` when testing for `netinet/if_ether.h`
to detect the Kernel/libc header conflict on musl.

After this patch, configure will correctly detect `netinet/if_ether.h` as
unusable and the subsequent compilation will not attempt to use it.

Fixes the following compatibility error:

	In file included from .../staging_dir/toolchain-mips_mips32_gcc-4.8-linaro_musl-1.1.9/include/linux/if_tun.h:20:0,
	                 from linux/device.c:24:
	.../staging_dir/toolchain-mips_mips32_gcc-4.8-linaro_musl-1.1.9/include/linux/if_ether.h:137:8: error: redefinition of 'struct ethhdr'
	 struct ethhdr {
	        ^
	In file included from .../staging_dir/toolchain-mips_mips32_gcc-4.8-linaro_musl-1.1.9/include/net/ethernet.h:10:0,
	                 from linux/../have.h:180,
	                 from linux/../system.h:26,
	                 from linux/device.c:21:
	.../staging_dir/toolchain-mips_mips32_gcc-4.8-linaro_musl-1.1.9/include/netinet/if_ether.h:96:8: note: originally defined here
	 struct ethhdr {
	        ^
	make[5]: *** [linux/device.o] Error 1

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