clang: suppress warnings

    lib/expr.c:4178:63: warning: passing 'uint32_t *'
          (aka 'unsigned int *') to parameter of type 'int *' converts between
          pointers to integer types with different sign [-Wpointer-sign]
                  if (grn_column_index(ctx, *p, c->op, &index, 1, &sid)) {
                                                                  ^~~~
    include/groonga.h:1854:69: note: passing argument
          to parameter 'section' here
                                 grn_obj **indexbuf, int buf_size, int *section);
                                                                        ^
    lib/expr.c:4183:63: warning: passing 'uint32_t *'
          (aka 'unsigned int *') to parameter of type 'int *' converts between
          pointers to integer types with different sign [-Wpointer-sign]
                  if (grn_column_index(ctx, *p, c->op, &index, 1, &sid)) {
                                                                  ^~~~
    include/groonga.h:1854:69: note: passing argument
          to parameter 'section' here
                                 grn_obj **indexbuf, int buf_size, int *section);
                                                                        ^