[gn build] Port cb647ec

Author: llvmgnsyncbot

llvm/utils/gn/secondary/llvm/tools/llvm-remarkutil/BUILD.gn

@@ -10,6 +10,7 @@ executable("llvm-remarkutil") {
     "RemarkConvert.cpp",
     "RemarkCount.cpp",
     "RemarkCounter.cpp",
+    "RemarkInstructionMix.cpp",
     "RemarkSizeDiff.cpp",
     "RemarkUtil.cpp",
     "RemarkUtilHelpers.cpp",