dma-resv: Fix undefined symbol when CONFIG_DMA_SHARED_BUFFER is disabled

When building with LLVM=1 for architectures like powerpc where
CONFIG_DMA_SHARED_BUFFER is not enabled, the build fails with:

  ld.lld: error: undefined symbol: dma_resv_reset_max_fences
  >>> referenced by helpers.c
  >>>               rust/helpers/helpers.o:(rust_helper_dma_resv_unlock)

The issue occurs because:
1. CONFIG_DEBUG_MUTEXES=y is enabled
2. CONFIG_DMA_SHARED_BUFFER is not enabled
3. dma_resv_reset_max_fences() is declared in the header when
   CONFIG_DEBUG_MUTEXES is set
4. But the function is only compiled in drivers/dma-buf/dma-resv.c,
   which is only built when CONFIG_DMA_SHARED_BUFFER is enabled
5. Rust helpers call dma_resv_unlock() which calls
   dma_resv_reset_max_fences(), causing an undefined symbol

Fix this by compiling `dma-resv.c` file only when CONFIG_DMA_SHARED_BUFFER
is enabled.

Fixes: 9b836641d3 ("rust: helpers: Add bindings/wrappers for dma_resv_lock")
Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Gary Guo <gary@garyguo.net>
Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20260708082454.1254320-3-mkchauras@gmail.com

rust/helpers/helpers.c

@@ -58,7 +58,9 @@
 #include "cred.c"
 #include "device.c"
 #include "dma.c"
+#ifdef CONFIG_DMA_SHARED_BUFFER
 #include "dma-resv.c"
+#endif
 #include "drm.c"
 #include "drm_gpuvm.c"
 #include "err.c"