remove using namespace std and use std namespace explicitly (#2125)

Removes `using namespace std` and adds `std::` explicitly instead, which
is usually on calls to `min`.

This is generally best practice, and it also might be helpful when there
are the same function names in the std namespace and in the global
namespace (e.g. #1833).

test_conformance/api/test_clone_kernel.cpp

@@ -20,8 +20,6 @@
 #include <string>
 #include <cmath>
 
-using namespace std;
-
 const char *clone_kernel_test_img[] =
 {
     "__kernel void img_read_kernel(read_only image2d_t img, sampler_t sampler, __global int* outbuf)\n"