Add 'void' to function definition to avoid compiler warnings
    
    During compiling of FreeIPA we are receiving compiler error:
    
    slapi-plugin.h:6149, GNU C Compiler 4 (gcc), Priority: Normal
    function declaration isn't a prototype [-Wstrict-prototypes]
    
    This patch should fix the compiler warning.
    
    Reviewed by nhosoi@redhat.com.