package/ell: fix build with older toolchains

package/ell: fix build with older toolchains

Older toolchains need to include sys/types.h and sys/socket.h before
linux/if.h, RTA_PREF was introduces with linux-4.1.x.

Fixes:

  http://autobuild.buildroot.net/results/2d1/2d1a3f82abb8475d39908b22f775c2dac781f330

  In file included from ell/rtnl.c:28:0:
  .../host/arm-buildroot-linux-gnueabi/sysroot/usr/include/linux/if.h:185:19: error: field 'ifru_addr' has incomplete type
  .../host/arm-buildroot-linux-gnueabi/sysroot/usr/include/linux/if.h:186:19: error: field 'ifru_dstaddr' has incomplete type
  .../host/arm-buildroot-linux-gnueabi/sysroot/usr/include/linux/if.h:187:19: error: field 'ifru_broadaddr' has incomplete type
  .../host/arm-buildroot-linux-gnueabi/sysroot/usr/include/linux/if.h:188:19: error: field 'ifru_netmask' has incomplete type
  .../arm-buildroot-linux-gnueabi/sysroot/usr/include/linux/if.h:189:20: error: field 'ifru_hwaddr' has incomplete type
  ell/rtnl.c: In function 'l_rtnl_route_extract':
  ell/rtnl.c:120:8: error: 'RTA_PREF' undeclared (first use in this function)

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>