Move gcc to general

Lot of packages explicitly requires gcc now, but almost all of
the components needs it.

Always keep gcc in hand-range.

files/rpms/general: always installs the libxlst2-devel, not just on
RHEL 6, package recompile capability always handy and these are small
packages anyway.

Change-Id: Ia7d0e1938fb2453e3c02b979f99d6d179c6a24c6
Merge branch 'master' of github.com:openstack-dev/devstack