fix breakage from last commit: forgot to include ksigaction.h

this file can be overridden by a same-named file in an arch dir.

src/internal/ksigaction.h

@@ -0,0 +1,6 @@
+struct k_sigaction {
+	void (*handler)(int);
+	unsigned long flags;
+	void (*restorer)(void);
+	long mask[2];
+};