UWP: Implement GetPathSeparatorForEnvironmentVar

Bug: angleproject:4142
Change-Id: Idb5e1b540d9ac7ab802357f247b41a6546173a35
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1925254
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

src/common/system_utils_winuwp.cpp

@@ -33,6 +33,12 @@ std::string GetEnvironmentVar(const char *variableName)
     return "";
 }
 
+const char *GetPathSeparatorForEnvironmentVar()
+{
+    // Not supported for UWP
+    return "";
+}
+
 const char *GetSharedLibraryExtension()
 {
     return "dll";