[OpenMP] Fix small merge artifact in OpenMP device runtime (#568)

We're currently using a variable/parameter that no longer exists in the
scope of the function, so we get a compilation error. Fix to align with
the upstream PR and use Ptr.

Author: agozillon

openmp/device/src/Allocator.cpp

@@ -98,7 +98,7 @@ void allocator::free(void *Ptr) {
 #elif defined(__AMDGPU__) && !defined(OMPTARGET_HAS_LIBC)
   __ockl_dm_dealloc(reinterpret_cast<uint64_t>(Ptr));
 #else
-  ::free(Size);
+  ::free(Ptr);
 #endif
 }