RV64: Include backend only once in multilevel build

When multiple parameters sets are bundled into a single build,
the RV64 backend should only be included once. This commit adjusts
the compilation guard in `native/riscv64/src/rv64v_poly.c accordingly.

Signed-off-by: Hanno Becker <beckphan@amazon.co.uk>

Author: hanno-becker

mlkem/native/riscv64/src/rv64v_poly.c

@@ -7,7 +7,8 @@
 
 #include "../../../common.h"
 
-#if defined(MLK_ARITH_BACKEND_RISCV64)
+#if defined(MLK_ARITH_BACKEND_RISCV64) && \
+    !defined(MLK_CONFIG_MULTILEVEL_NO_SHARED)
 
 #include <riscv_vector.h>
 #include "arith_native_riscv64.h"
@@ -806,11 +807,11 @@ unsigned int mlk_rv64v_rej_uniform(int16_t *r, unsigned int len,
   return ctr;
 }
 
-#else /* MLK_ARITH_BACKEND_RISCV64 */
+#else /* MLK_ARITH_BACKEND_RISCV64 && !MLK_CONFIG_MULTILEVEL_NO_SHARED */
 
 MLK_EMPTY_CU(rv64v_poly)
 
-#endif /* !MLK_ARITH_BACKEND_RISCV64 */
+#endif /* !(MLK_ARITH_BACKEND_RISCV64 && !MLK_CONFIG_MULTILEVEL_NO_SHARED) */
 
 /* To facilitate single-compilation-unit (SCU) builds, undefine all macros.
  * Don't modify by hand -- this is auto-generated by scripts/autogen. */