Kernel: Add missing include in API

This remained undetected for a long time as HeaderCheck is disabled by
default. This commit makes the following file compile again:

    // file: compile_me.cpp
    #include <Kernel/API/POSIX/ucontext.h>
    // That's it, this was enough to cause a compilation error.

Kernel/API/POSIX/ucontext.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <Kernel/API/POSIX/signal.h>
 #include <Kernel/API/POSIX/sys/types.h>
 #include <Kernel/Arch/mcontext.h>