mk: bump the shared object major version to 2.

This is to indicate the ABI breakage in the bdev library. A function's
argument list was changed which breaks both backwards and forwards
compatibility.

Going forward, all backwards compatibility breaking changes should be
marked with a rev of the SO major version for that library. All forwards
compatibility breaking changes should be marked with a rev of the SO
minor version.

Change-Id: I35e45c102c5c6de3c684919a10e5116f8f2c375f
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/1066


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Reviewed-by: Aleksey Marchuk <alexeymar@mellanox.com>