libaio: harness: fix build error due to linker search order
    
    Move '-lpthread' after 'main.c' in the compiler command line, so that
    the linker can search unresolved symbols from the test in the library.
    
        cases/18.t:98: undefined reference to `pthread_create'
        cases/18.t:103: undefined reference to `pthread_join'
        collect2: error: ld returned 1 exit status
    
    Signed-off-by: Mauricio Faria de Oliveira <mauricfo@linux.vnet.ibm.com>
    Signed-off-by: Jeff Moyer <jmoyer@redhat.com>