net/e1000: fix build of igb only

net/e1000: fix build of igb only

This commit fixes a compilation error if EM_PMD is
not defined, bug IGB_PMD is. The root cause of the
issue was that log init variables are declared as
extern in a header file, while the definition of the
variables was in e1000_ethdev.c. Hence, the definitions
were not available if the e1000 PMD is disabled.

To fix this, a new file is added e1000_logs.c, which
matches the e1000_logs.h header. The log variables are
always compiled in, but the PMD logs are only registered
if a PMD is enabled in the configuration. Extra checks
are added in order to avoid duplicate registering.

Fixes: ed5bbb767c3e ("net/e1000: implement dynamic logging")
Cc: stable@dpdk.org
Reported-by: Vipin Varghese <vipin.varghese@intel.com>
Signed-off-by: Harry van Haaren <harry.van.haaren@intel.com>
Acked-by: Vipin Varghese <vipin.varghese@intel.com>