Limit the scope of gpu_rand_generator (AMReX-Codes#3659)

Move gpu_rand_generator from namespace amrex to anonymous namespace. The
variable is only used in AMReX_Random.cpp, so there are no reasons for
it to be in the amrex namespace.

Src/Base/AMReX_Random.cpp

@@ -19,9 +19,15 @@ namespace
 namespace amrex {
 #ifdef AMREX_USE_SYCL
  sycl_rng_descr* rand_engine_descr = nullptr;
- oneapi::mkl::rng::philox4x32x10* gpu_rand_generator = nullptr;
 #else
  amrex::randState_t* gpu_rand_state = nullptr;
+#endif
+}
+
+namespace {
+#ifdef AMREX_USE_SYCL
+ oneapi::mkl::rng::philox4x32x10* gpu_rand_generator = nullptr;
+#else
  amrex::randGenerator_t gpu_rand_generator = nullptr;
 #endif
 }