Define XT_DS_RESOURCE_DIR in the Linux Makefile.

XT_DS_RESOURCE_DIR was hard-coded to /usr/local/share/xombrero.  If a
user ran make with a custom PREFIX, XT_DS_RESOURCE_DIR was not accurate.

Note: the change has only been made to the Linux Makefile.  I'll happily add the change to the other Makefiles but would like some feedback first as I'm not very familiar with xombrero's structure and policies.