[gn build] Port 6723165

Author: llvmgnsyncbot

llvm/utils/gn/secondary/lldb/source/Plugins/ABI/X86/BUILD.gn

@@ -16,5 +16,6 @@ static_library("X86") {
     "ABISysV_x86_64.cpp",
     "ABIWindows_x86_64.cpp",
     "ABIX86.cpp",
+    "ABIX86_i386.cpp",
   ]
 }