package/seatd: fix build with gcc < 7

package/seatd: fix build with gcc < 7

Since its addition in commit daae31149017cbf8b03634cfc56da132f7e1ad83,
seatd fails to build with gcc < 7 because it unconditonally uses
-Wimplicit-fallthrough which is only available since gcc 7.1 and
https://github.com/gcc-mirror/gcc/commit/81fea426da8c4687bb32e6894dc26f00ae211822
resulting in the following build failure:

arm-none-linux-gnueabi-gcc: error: unrecognized command line option '-Wimplicit-fallthrough'

Fixes:
 - http://autobuild.buildroot.org/results/0ee6816a7cceebdafd07612677a594bdf68e0790

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>