libmemif: fix gcc strict-proto error

libmemif: fix gcc strict-proto error

- Changing function prototypes for a couple libmemif functions so that
  gcc will successfully compile with the "-Wstrict-prototypes" flag
  enabled.
- Re-ran unit tests via "make test" locally on Ubuntu 20.04 LTS x64.
  One issue (likely unrelated to patch):
  * Testcase name: Bidirectional Forwarding Detection (BFD)
    ERROR: put session admin-up and admin-down
    [test_bfd.BFD4TestCase.test_admin_up_down]

Type: fix
Change-Id: Ibcb7210873d39aec9348fe8ac7eebc78d089d8e0
Signed-off-by: Matthew Giassa <mgiassa@cisco.com>