net/nfb: fix dependency check

net/nfb: fix dependency check

The library libnfb is part of netcope-common which provides
a pkg-config file: netcope-common.pc.
Looking for this .pc file - with dependency() - is preferred
than looking for the library - with cc.find_library().

If the library is not installed in a standard path,
it can be found thanks to PKG_CONFIG_PATH variable.
The previous solution required to use CFLAGS and LDFLAGS
environment variables.

Fixes: 6435f9a0ac22 ("net/nfb: add new netcope driver")
Cc: stable@dpdk.org
Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Jan Remes <remes@netcope.com>