build: add libbsd to pkg-config file if enabled

build: add libbsd to pkg-config file if enabled

If libbsd is enabled in DPDK, the strlcpy and strlcat functions in
rte_string_fns.h redirect to the varients in libbsd, only using the
fallbacks if it is not enabled. Therefore, if libbsd is enabled, it needs
to be called out as a DPDK dependency in the pkgconfig file.

To ensure that we don't have undefined variables on non-Linux platforms, we
can remove the linux condition around the libbsd check - no harm comes in
looking for it on other OS, since it's an optional dependency.

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