Conditionally use SIGSTKFLT

Change-Id: I555b33b5a93df332c556944a1c00fff417825247
diff --git a/toolbox/kill.c b/toolbox/kill.c
index b79805f..fa2f649 100644
--- a/toolbox/kill.c
+++ b/toolbox/kill.c
@@ -42,7 +42,9 @@
     /* non-SUS signals */
     _SIG(IO),
     _SIG(PWR),
+#ifdef SIGSTKFLT
     _SIG(STKFLT),
+#endif
     _SIG(WINCH),
 #undef _SIG
 };