package/turbolua: Fix build with gcc7

package/turbolua: Fix build with gcc7

gcc7 has -Wimplicit-fallthrough enabled by default. Together with
-Werror in turbolua makefile it gives -Werror=implicit-fallthrough=
build error.

Add upstreamable patch [1], which fixes typo in FALLTHROUGH comment,
so it makes the fallthrough explicit for gcc.

[1] https://github.com/kernelsauce/turbo/pull/323

Fixes:
http://autobuild.buildroot.net/results/6f107b62f105508044185576b071323e5d73dd83
http://autobuild.buildroot.net/results/d303929960c667a21181d0bbdab64ba8a8d56a4f

Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>