CP-9987: Add blktap-devel as a dependency to xcp-rrdd

As part of the blktap3 stats changes, make xcp-rrdd build depend on the
blktap-devel RPM which contains the required headers.

Signed-off-by: Siddharth Vinothkumar <siddharth.vinothkumar@citrix.com>
Merge pull request #97 from siddharthv/CP-9987

CP-9987: Add blktap-devel as a dependency to xcp-rrdd