lib/nolibc: Import `signalfd` definitions from `musl`

lib/nolibc: Import `signalfd` definitions from `musl`

Import `musl`'s header for `signalfd` definitions to allow
nolibc builds a way to call into the signalfd implementation.

Source of import:
Repository: https://git.musl-libc.org/cgit/musl
Tag: v1.2.4 (f5f55d65)
Path: include/sys/signalfd.h

Checkpatch-Ignore: SPDX_LICENSE_TAG
Checkpatch-Ignore: FUNCTION_ARGUMENTS
Checkpatch-Ignore: PREFER_KERNEL_TYPES
Checkpatch-Ignore: SPACING
Checkpatch-Ignore: COMMIT_LOG_USE_LINK
Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Approved-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Reviewed-by: Andrei Tatar <andrei@unikraft.io>
GitHub-Closes: #1619