[gn build] Add missed comma

Author: aeubanks

llvm/utils/gn/secondary/llvm/lib/Target/AMDGPU/BUILD.gn

@@ -131,7 +131,7 @@ static_library("LLVMAMDGPUCodeGen") {
     "AMDGPUAttributor.cpp",
     "AMDGPUCallLowering.cpp",
     "AMDGPUCodeGenPrepare.cpp",
-    "AMDGPUCombinerHelper.cpp"
+    "AMDGPUCombinerHelper.cpp",
     "AMDGPUCtorDtorLowering.cpp",
     "AMDGPUExportClustering.cpp",
     "AMDGPUFixFunctionBitcasts.cpp",