pbx_variables: add missing ASTSBINDIR variable

pbx_variables: add missing ASTSBINDIR variable

Every config variable in the directories
section of asterisk.conf currently has a
counterpart built-in variable containing
the value of the config option, except
for the last one, astsbindir, which should
have an ASTSBINDIR variable.

However, the actual corresponding ASTSBINDIR
variable is missing in pbx_variables.c.

This adds the missing variable so that all
the config options have their corresponding
variable.

ASTERISK-29847 #close

Change-Id: I36006faf471825b36ebc8aa5e87a3bcb38d446fc