[RISCV] Include RISCVGenSearchTable.inc in RISCVISelDAGToDAG.h (#126326)

This line was previously removed when
12d4724 moved it to RISCVInstrInfo.h.
But we probably don't want to have dangling `#define *_DECL`
(RISCVGenSearchableTables.inc will `#undef` these macros) and I think
there is no harm putting declarations of those search table functions in
RISCVISelDAGToDAG.h.

Author: mshockwave

llvm/lib/Target/RISCV/RISCVISelDAGToDAG.h

@@ -278,6 +278,7 @@ struct VLX_VSXPseudo {
 #define GET_RISCVVSETable_DECL
 #define GET_RISCVVLXTable_DECL
 #define GET_RISCVVSXTable_DECL
+#include "RISCVGenSearchableTables.inc"
 } // namespace RISCV
 
 } // namespace llvm