LAGraph: Link to OpenMP::OpenMP_C target

[mmuetzel]

Building LAGraph with OpenMP currently fails if the OpenMP headers are installed in a non-default location.
E.g.: https://github.com/DrTimothyAldenDavis/SuiteSparse/actions/runs/6578211130/job/17871438146#step:7:6548

  [  0%] Building C object src/CMakeFiles/lagraph.dir/algorithm/LAGr_Betweenness.c.o
  In file included from /Users/runner/work/SuiteSparse/SuiteSparse/LAGraph/src/algorithm/LAGr_Betweenness.c:79:
  In file included from /Users/runner/work/SuiteSparse/SuiteSparse/LAGraph/src/utility/LG_internal.h:30:
  /Users/runner/work/SuiteSparse/SuiteSparse/LAGraph/include/LAGraph.h:51:14: fatal error: 'omp.h' file not found
      #include <omp.h>
               ^~~~~~~
  1 error generated.

The proposed change hopefully fixes that by linking to the imported CMake target OpenMP::OpenMP_C which should be providing all necessary information.

(Again, it would be easier to check whether this works if #450 was merged.)

[DrTimothyAldenDavis]

Thanks! I've merged #450, and I'll merge this one now.