files/slackpkg: replace #!/bin/sh with #!/bin/bash

From Eugen Wissner:
/usr/sbin/slackpkg claims to be a /bin/sh script, but it is not, since
it contains bashisms (bash arrays in the configuration, bash functions).

files/slackpkg

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # SlackPkg - An Automated packaging tool for Slackware Linux
 # Copyright (C) 2003-2011 Roberto F. Batista, Evaldo Gardenali