[gn] Make RISCV/MCTargetDesc depend on RISCV/TargetInfo

RISCVMCTargetDesc.cpp calls getTheRISCV{32,64}Target(), which is in TargetInfo.
Looks like this dependency has been missing for a long time.

llvm/utils/gn/secondary/llvm/lib/Target/RISCV/MCTargetDesc/BUILD.gn

@@ -58,6 +58,7 @@ static_library("MCTargetDesc") {
     "//llvm/lib/MC",
     "//llvm/lib/Support",
     "//llvm/lib/Target/RISCV:RISCVGenCompressInstEmitter",
+    "//llvm/lib/Target/RISCV/TargetInfo",
     "//llvm/lib/TargetParser",
   ]
   include_dirs = [ ".." ]