patch 9.0.0287: Irix systems no longer exist

Problem:    Irix systems no longer exist.
Solution:   Remove references to Irix. (Yegappan Lakshmanan, closes #10994)
diff --git a/src/os_unix.c b/src/os_unix.c
index 7aa3d1f..4dadac3 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -252,7 +252,7 @@
 /*
  * I have seen
  *  extern char *_sys_siglist[NSIG];
- * on Irix, Linux, NetBSD and Solaris. It contains a nice list of strings
+ * on Linux, NetBSD and Solaris. It contains a nice list of strings
  * that describe the signals. That is nearly what we want here.  But
  * autoconf does only check for sys_siglist (without the underscore), I
  * do not want to change everything today.... jw.