INSTALL.md: Add 'which' as build dependency

Update INSTALL.md to list 'which' as a required dependency.
Without it, scst init script fails with:

    /etc/init.d/scst: line 149: which: command not found

as seen in systemd logs:

    mp-scst-service scst[1198]: /etc/init.d/scst: line 149: which: command not found
    mp-scst-service systemd[1]: mp-scst.service: Control process exited, code=exited, status=1/FAILURE

Stemming from the line:
https://github.com/SCST-project/scst/blob/v3.9/scstadmin/init.d/scst#L162

INSTALL.md

@@ -4,6 +4,7 @@
 
 If the following packages have not yet been installed, install these now:
 
+    which
     bzip2
     gcc
     kernel-devel or kernel-default-devel