Fix debug compile error in gather_struct_tests.cpp

[davidwendt]

Building libcudf in debug resulted in a compile error introduced by #8527

../tests/copying/gather_struct_tests.cpp:87:32: error: suggest parentheses around ‘&&’ within ‘||’ [-Werror=parentheses]
   87 |       assert(gather_index >= 0 && gather_index < gather_map.size() ||
      |              ~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This error only shows in a debug build. This PR fixes the this code by applying the parentheses as the error suggests.

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@@               Coverage Diff               @@
##             branch-21.08    #8554   +/-   ##
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[ttnghia]

This is duplicate of #8565

[vuule]

@gpucibot merge