client: Fix build break on Windows.

client/gpu_nvidia.cpp

@@ -47,6 +47,8 @@ using std::string;
 
 static void get_available_nvidia_ram(COPROC_NVIDIA &cc, vector<string>& warnings);
 
+#ifndef _WIN32
+
 static int nvidia_driver_version() {
     int (*nvml_init)()  = NULL;
     int (*nvml_finish)()  = NULL;
@@ -73,6 +75,8 @@ end:
     return dri_ver;
 }
 
+#endif 
+
 // return 1/-1/0 if device 1 is more/less/same capable than device 2.
 // factors (decreasing priority):
 // - compute capability