Implement SDL_GetPath stub for all OSes

src/filesystem/android/SDL_sysfilesystem.c

@@ -52,4 +52,12 @@ SDL_GetPrefPath(const char *org, const char *app)
     return NULL;
 }
 
+char *SDL_GetPath(SDL_Folder folder)
+{
+    /* TODO: see https://developer.android.com/reference/android/os/Environment#lfields
+       and https://stackoverflow.com/questions/39332085/get-path-to-pictures-directory */
+    SDL_Unsupported();
+    return NULL;
+}
+
 #endif /* SDL_FILESYSTEM_ANDROID */