[flang][CUF] fix missing header after #112188 (#118993)

Otherwise, builds with `-DFLANG_CUF_RUNTIME` hits:

```
runtime/CUDA/descriptor.cpp:44:24: error: invalid use of incomplete type 'const class Fortran::runtime::Descriptor'
   44 |   std::size_t count{src->SizeInBytes()};
```

Author: jeanPerier

flang/runtime/CUDA/descriptor.cpp

@@ -10,6 +10,7 @@
 #include "../terminator.h"
 #include "flang/Runtime/CUDA/allocator.h"
 #include "flang/Runtime/CUDA/common.h"
+#include "flang/Runtime/descriptor.h"
 
 #include "cuda_runtime.h"