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

lib/gpu/lal_ufm.cpp

@@ -25,7 +25,7 @@ const char *ufm=0;
 
 #include "lal_ufm.h"
 #include <cassert>
-using namespace LAMMPS_AL;
+namespace LAMMPS_AL {
 #define UFMT UFM<numtyp, acctyp>
 
 extern Device<PRECISION,ACC_PRECISION> device;
@@ -170,3 +170,4 @@ void UFMT::loop(const bool _eflag, const bool _vflag) {
 }
 
 template class UFM<PRECISION,ACC_PRECISION>;
+}