'null_logtype_driver' global variable is defined in a header file which
was causing multiple definitions of the variable, fixed it by moving it
to the .c file.
Issue has been detected by '-fno-common' gcc flag.
Fixes: 735b783d8c2b ("crypto/null: add dynamic logging")
Cc: stable@dpdk.org
Signed-off-by: Ferruh Yigit <ferruh.yigit@intel.com>