test-trailer: test1(): fix paren placement

t/test-trailer.c:56:46: warning: size argument in 'memcmp' call is a comparison
      [-Wmemsize-comparison]
        if (memcmp(&t1, &t2, sizeof(struct trailer) != 0)) {
                             ~~~~~~~~~~~~~~~~~~~~~~~^~~~
t/test-trailer.c:56:6: note: did you mean to compare the result of 'memcmp' instead?
        if (memcmp(&t1, &t2, sizeof(struct trailer) != 0)) {
            ^                                           ~
                                                   )
t/test-trailer.c:56:23: note: explicitly cast the argument to size_t to silence this warning
        if (memcmp(&t1, &t2, sizeof(struct trailer) != 0)) {
                             ^
                             (size_t)(                  )