fix: adding signal.h to pass build for macOS

[jgabaut]

• Tested on Linux and macOS.
• Should solve the missing kill() rambling from the compiler.
• Closes #110.
• Drop TODO about testing build on macOS after ensuring the build works.

[rexim]

Thank you! Fixed by 76062c05a299069125c419e1fee0fdab69dc09ec