libedit changed it's interface a while ago. MariaDB's cmake file doesn't
recognize the new interface, the compile test fails:

/mariadb-10.2.19/CMakeFiles/CMakeTmp/src.cxx: In function 'int main(int, char**)':
/mariadb-10.2.19/CMakeFiles/CMakeTmp/src.cxx:6:47: error: invalid conversion from 'char*' to 'int' [-fpermissive]
  int res= (*rl_completion_entry_function)(0,0);
           ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~

Fix this by adding a detection for the new interface as well.

Run-tested on a MIPS machine.
Signed-off-by: Sebastian Kemper <sebastian_ml@gmx.net>