[gn build] Port 50949eb

Author: llvmgnsyncbot

llvm/utils/gn/secondary/lldb/source/API/BUILD.gn

@@ -80,6 +80,7 @@ target(liblldb_type, "liblldb") {
     "SBMemoryRegionInfoList.cpp",
     "SBModule.cpp",
     "SBModuleSpec.cpp",
+    "SBMutex.cpp",
     "SBPlatform.cpp",
     "SBProcess.cpp",
     "SBProcessInfo.cpp",