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>

extras/libmemif/src/libmemif.h

@@ -477,12 +477,12 @@ typedef struct
 
     \return ((MEMIF_VERSION_MAJOR << 8) | MEMIF_VERSION_MINOR)
 */
-uint16_t memif_get_version ();
+uint16_t memif_get_version (void);
 
 /** \brief Get memif version as string
     \return major.minor
 */
-const char *memif_get_version_str ();
+const char *memif_get_version_str (void);
 
 /** \brief Memif get queue event file descriptor
     @param conn - memif connection handle
@@ -771,4 +771,4 @@ int memif_set_connection_request_timer (memif_socket_handle_t sock,
 
 /** @} */
 
-#endif /* _LIBMEMIF_H_ */
+#endif /* _LIBMEMIF_H_ */