make clang++ happy when trying to compile the GPU library

lib/gpu/lal_base_three.cpp

@@ -14,7 +14,7 @@
  ***************************************************************************/
 
 #include "lal_base_three.h"
-using namespace LAMMPS_AL;
+namespace LAMMPS_AL {
 #define BaseThreeT BaseThree<numtyp, acctyp>
 
 extern Device<PRECISION,ACC_PRECISION> global_device;
@@ -397,4 +397,4 @@ void BaseThreeT::compile_kernels(UCL_Device &dev, const void *pair_str,
 }
 
 template class BaseThree<PRECISION,ACC_PRECISION>;
-
+}