Configuration error when BUILD_GPU=ON

[wxianghao]

The cmake configuration fails when enabling BUILD_GPU due to mixed up uppercase and lowercase of the source file name opengjk_gpu.cu.

To reproduce the issue:

cmake -B build -DBUILD_GPU=ON

The error:

CMake Error at gpu/CMakeLists.txt:81 (add_library):
  Cannot find source file:

    openGJK_GPU.cu

  Tried extensions .c .C .c++ .cc .cpp .cxx .cu .mpp .m .M .mm .ixx .cppm
  .ccm .cxxm .c++m .h .hh .h++ .hm .hpp .hxx .in .txx .f .F .for .f77 .f90
  .f95 .f03 .hip .ispc


CMake Error at gpu/CMakeLists.txt:81 (add_library):
  No SOURCES given to target: openGJK_GPU

[wxianghao]

I have added a PR to fix it, please check #71