Add missing namespace to PPCLinux.cpp

This fixes a build failure with MSVC introduced in
https://reviews.llvm.org/D112906

Reviewed By: nemanjai

Differential Revision: https://reviews.llvm.org/D118211

clang/lib/Driver/ToolChains/PPCLinux.cpp

@@ -13,6 +13,7 @@
 #include "llvm/Support/FileSystem.h"
 #include "llvm/Support/Path.h"
 
+using namespace clang::driver;
 using namespace clang::driver::toolchains;
 using namespace llvm::opt;
 using namespace llvm::sys;