Gate the SDMA factory source on USE_SDMA, like its implementation

src/CMakeLists.txt guards the SDMA backend with `if (USE_SDMA)
add_subdirectory(sdma) endif()`, so src/sdma/anvil.cpp -- which defines
everything in namespace sdma_anvil -- is only compiled when USE_SDMA is on.
But sdma/gin_anvil_sdma_factory.cpp, which CALLS into that namespace, sits in
the unconditional source list above.

With upstream's own default of USE_SDMA=OFF the library therefore fails to
link:

  ld: librocshmem.so: undefined reference to `sdma_anvil::initEndpoint()'
  ld: librocshmem.so: undefined reference to `sdma_anvil::anvil'
  ld: librocshmem.so: undefined reference to `sdma_anvil::SdmaQueue::deviceHandle() const'
  ... and more

Move the factory next to the implementation it depends on, so both are
governed by the same switch.

--- a/src/CMakeLists.txt
+++ b/src/CMakeLists.txt
@@ -46,7 +46,6 @@
     wf_coal_policy.cpp
     ipc_policy.cpp
     sdma_policy.cpp
-    sdma/gin_anvil_sdma_factory.cpp
 )

 set(
--- a/src/sdma/CMakeLists.txt
+++ b/src/sdma/CMakeLists.txt
@@ -28,4 +28,5 @@
   ${PROJECT_NAME}
   PRIVATE
     anvil.cpp
+    gin_anvil_sdma_factory.cpp
 )