Include ucontext header files

Include ucontext header files

While musl doesn't provide an actual implementation for the legacy
ucontext API, the header is still useful to compile software against
musl. Also, some software only relies on the definitions in the header.

Signed-off-by: Marco Schlumpp <marco@unikraft.io>
Reviewed-by: Eduard Vintilă <eduard.vintila47@gmail.com>
Reviewed-by: Sergiu Moga <sergiu.moga@protonmail.com>
Approved-by: Razvan Deaconescu <razvand@unikraft.io>
Tested-by: Unikraft CI <monkey@unikraft.io>
GitHub-Closes: #41