[SYCL][NFC] Fix compilation warnings

[fwyzard]

Fix two compilation warnings:

.../sycl/plugins/cuda/pi_cuda.cpp: In function 'pi_result piPluginInit(pi_plugin*)':
.../sycl/plugins/cuda/pi_cuda.cpp:3491:73: error: cast between incompatible function types from 'pi_result (*)(pi_device, pi_device_binary_struct**, pi_uint32, pi_device_binary_struct**)' {aka '_pi_result (*)(_pi_device*, pi_device_binary_struct**, unsigned int, pi_device_binary_struct**)'} to 'pi_result (*)(pi_device, pi_program, const char*, pi_uint64*)' {aka '_pi_result (*)(_pi_device*, _pi_program*, const char*, long unsigned int*)'} [-Werror=cast-function-type]
   (PluginInit->PiFunctionTable).pi_api = (decltype(&::pi_api))(&cuda_api);
                                                                         ^
.../sycl/plugins/cuda/pi_cuda.cpp:3504:3: note: in expansion of macro '_PI_CL'
   _PI_CL(piextGetDeviceFunctionPointer, cuda_piextGetDeviceFunctionPointer)
   ^~~~~~

and

.../sycl/plugins/opencl/pi_opencl.cpp: In function 'pi_result piPluginInit(pi_plugin*)':
.../sycl/plugins/opencl/pi_opencl.cpp:1028:72: error: cast between incompatible function types from 'pi_result (*)(pi_queue, pi_bool, void*, const void*, pi_int32, pi_uint32, _pi_event* const*, _pi_event**)' {aka '_pi_result (*)(_pi_queue*, unsigned int, void*, const void*, int, unsigned int, _pi_event* const*, _pi_event**)'} to 'pi_result (*)(pi_queue, pi_bool, void*, const void*, size_t, pi_uint32, _pi_event* const*, _pi_event**)' {aka '_pi_result (*)(_pi_queue*, unsigned int, void*, const void*, long unsigned int, unsigned int, _pi_event* const*, _pi_event**)'} [-Werror=cast-function-type]
   (PluginInit->PiFunctionTable).pi_api = (decltype(&::pi_api))(&ocl_api);
                                                                        ^
.../sycl/plugins/opencl/pi_opencl.cpp:1120:3: note: in expansion of macro '_PI_CL'
   _PI_CL(piextUSMEnqueueMemcpy, OCL(piextUSMEnqueueMemcpy))
   ^~~~~~

[fwyzard]

The first is for a non-implemented function (piextGetDeviceFunctionPointer) whose prototype seems to have gotten out of sync.

The second is an inconsistency between pi_int32 (i.e. int) and size_t (i.e. unsigned long int).