lavu: add calling convention for OpenCL callback.

This fix a build error on Windows:
C2440: connot convert from 'void (__cdecl *) (...)' to 'void (__stdcall *)(...)'.
Signed-off-by: Ruiling Song <ruiling.song@intel.com>

libavutil/hwcontext_opencl.c

@@ -141,8 +141,9 @@ typedef struct OpenCLFramesContext {
 } OpenCLFramesContext;
 
 
-static void opencl_error_callback(const char *errinfo,
-                                  const void *private_info, size_t cb,
+static void CL_CALLBACK opencl_error_callback(const char *errinfo,
+                                              const void *private_info,
+                                              size_t cb,
                                               void *user_data)
 {
     AVHWDeviceContext *ctx = user_data;