LibCore: Add FreeBSD as OpenBSD/Apple in System.cpp & System.h

Author: https://github.com/iah 🔰
Commit: 4f716caa33
Pull-request: https://github.com/SerenityOS/serenity/pull/12521
Reviewed-by: https://github.com/MaxWipfli ✅
Reviewed-by: https://github.com/linusg
Reviewed-by: https://github.com/nico
Reviewed-by: https://github.com/trflynn89

Userland/Libraries/LibCore/System.cpp

@@ -221,7 +221,7 @@ ErrorOr<void> sigaction(int signal, struct sigaction const* action, struct sigac
     return {};
 }
 
-#if defined(__APPLE__) || defined(__OpenBSD__)
+#if defined(__APPLE__) || defined(__OpenBSD__) || defined(__FreeBSD__)
 ErrorOr<sig_t> signal(int signal, sig_t handler)
 #else
 ErrorOr<sighandler_t> signal(int signal, sighandler_t handler)

Userland/Libraries/LibCore/System.h

@@ -58,7 +58,7 @@ ErrorOr<int> accept4(int sockfd, struct sockaddr*, socklen_t*, int flags);
 #endif
 
 ErrorOr<void> sigaction(int signal, struct sigaction const* action, struct sigaction* old_action);
-#if defined(__APPLE__) || defined(__OpenBSD__)
+#if defined(__APPLE__) || defined(__OpenBSD__) || defined(__FreeBSD__)
 ErrorOr<sig_t> signal(int signal, sig_t handler);
 #else
 ErrorOr<sighandler_t> signal(int signal, sighandler_t handler);