src: Add missing PRIVATE keyword to target_link_libraries command.

src: Add missing PRIVATE keyword to target_link_libraries command.

This fixes the following cmake error that happens if you have libzbc installed:

CMake Error at src/blk/CMakeLists.txt:40 (target_link_libraries):
  The keyword signature for target_link_libraries has already been used with
  the target "blk".  All uses of target_link_libraries with a target must be
  either all-keyword or all-plain.

  The uses of the keyword signature are here:

   * src/blk/CMakeLists.txt:32 (target_link_libraries)

Signed-off-by: Abutalib Aghayev <agayev@cs.cmu.edu>