adds <string.h> includes for ROCm 7 compatibility

[frawanye]

Without these includes, compiling with rocm 7 causes the following error: no matching function for call to 'memcpy'
in the following files:

• src/gpu/assemble_MPI_vector_cuda.cu
• src/gpu/lts_assembly_mpi_cuda.cu
• src/gpu/prepare_mesh_constants_cuda.cu

[danielpeter]

thanks for the update!

as this didn't show up so far, what system were you using to compile the code?

anyway, i'll merge it and then probably move these include statements #include <string.h> to the file mesh_constants_gpu.h for convenience and add some HIP compilation checks.

thanks again,
-daniel