slang: fix build on musl

Add patch from Alpine Linux to fix the build of slang with the musl C
library.

Fixes:

  http://autobuild.buildroot.org/results/bc3/bc3cca6e6a906afa4c57dcddfb4a9ecd0cb44398/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>