configure.ac: dpdk: append DPDK_CFLAGS to LIBDPDK_CPPFLAGS

configure.ac: dpdk: append DPDK_CFLAGS to LIBDPDK_CPPFLAGS

In some way it is bad naming in pkg-config - it is always some sort of
conflated ldflags + libs and cppflags + cxxflags + cflags.
Due to that to work correctly we append DPDK_CFLAGS (if defined by
pkg-config) to LIBDPDK_CPPFLAGS as well.
This intentionally does not add those CFLAGS specified by a user to
allow full override and fully separated CLFAGS/CPPFLAGS if needed.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>