build: rename pkgconfig to libdpdk.pc

build: rename pkgconfig to libdpdk.pc

In Debian and Ubuntu we have been shipping a pkgconfig file for DPDK
for more than a year now, and the filename is libdpdk.pc.
A few downstream projects, like OVS and Collectd, have adopted the
use of libdpdk.pc in their build systems as well.
In order to maintain backward compatibility, rename the file from
DPDK.pc to libdpdk.pc.

Signed-off-by: Luca Boccassi <bluca@debian.org>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>