Buid error with gfortran version >= 10

[pkestene]

Complete error is

    6 | integer,private :: col(7) = [ Z'0000ff00', Z'000000ff', Z'00ffff00', Z'00ff00ff', Z'0000ffff', Z'00ff0000', Z'00ffffff']
      |                             1
Error: BOZ literal constant at (1) cannot appear in an array constructor
nvtx.f90:62:16:

   62 |     event%color=col(mod(id,7)+1)
      |                1
Error: Function ‘col’ at (1) has no IMPLICIT type
main.f90:2:7:

    2 |   use nvtx
      |       1
Fatal Error: Cannot open module file ‘nvtx.mod’ for reading at (1): No such file or directory
compilation terminated.

A fix is provided in PR #1