[OpenMP][NVPTX] Fixed missing variables for CUDA free compilation in NVPTX plugin

[tianshilei1992]

Summary

CU_EVENT_DEFAULT is defined in CUDA header. It should be added to
openmp/libomptarget/plugins/cuda/dynamic_cuda/cuda.h for CUDA free build.

openmp/libomptarget/plugins/cuda/dynamic_cuda/cuda.h

 } CUmemAttach_flags;
 
 typedef enum CUcomputeMode_enum {
   CU_COMPUTEMODE_DEFAULT = 0,
   CU_COMPUTEMODE_PROHIBITED = 2,
   CU_COMPUTEMODE_EXCLUSIVE_PROCESS = 3,
 } CUcompute_mode;
 
+typedef enum CUevent_flags_enum {
+  CU_EVENT_DEFAULT = 0x0,
+  CU_EVENT_BLOCKING_SYNC = 0x1,
+  CU_EVENT_DISABLE_TIMING = 0x2,
+  CU_EVENT_INTERPROCESS = 0x4
+} CUevent_flags;
+
 CUresult cuCtxGetDevice(CUdevice *);
 CUresult cuDeviceGet(CUdevice *, int);
 CUresult cuDeviceGetAttribute(int *, CUdevice_attribute, CUdevice);
 CUresult cuDeviceGetCount(int *);
 CUresult cuFuncGetAttribute(int *, CUfunction_attribute, CUfunction);
 
 // Device info
 CUresult cuDeviceGetName(char *, int, CUdevice);