Compile cdef public functions from torch_allocator with C ABI (#1350)

Since Cython 3, we must explicitly request a C ABI with `-DCYTHON_EXTERN_C='extern "C"'` for `cdef public` functions.

- Closes #1349

Before:
```
$ nm -D .../rmm/python/rmm/_lib/torch_allocator.cpython-310-x86_64-linux-gnu.so | grep allocate
00000000000071c0 T _Z10deallocatePvlS_
0000000000007500 T _Z8allocateliPv
...
```

After:
```
$ nm -D .../rmm/python/rmm/_lib/torch_allocator.cpython-310-x86_64-linux-gnu.so | grep allocate
0000000000007500 T allocate
00000000000071c0 T deallocate
...
```

Authors:
  - Lawrence Mitchell (https://github.com/wence-)

Approvers:
  - Ashwin Srinath (https://github.com/shwina)
  - Bradley Dice (https://github.com/bdice)

URL: #1350

python/rmm/_lib/CMakeLists.txt

@@ -19,3 +19,5 @@ set(linked_libraries rmm::rmm)
 # Build all of the Cython targets
 rapids_cython_create_modules(SOURCE_FILES "${cython_sources}" LINKED_LIBRARIES "${linked_libraries}"
                                                                                CXX)
+# The cdef public functions in this file need to have a C ABI
+target_compile_definitions(torch_allocator PRIVATE CYTHON_EXTERN_C=extern\ "C")