build: Use correct value of %dist in RPM and SRPM dependencies

RPMs are built in the mock chroot, so filenames in dependency
rules should use the value of %dist from the mock - typically
a CentOS value such as '.el6'.   SRPMs are built outside the
mock chroot, so their names will include the value of %dist
set by the host operating system - for instance '.fc18'.

Signed-off-by: Euan Harris <euan.harris@citrix.com>