Fix -Wformat-security warnings

These warnings are kinda spammy, and they make sense in principle.
vlog_error takes a format string, so passing it an arbitrary string
(even error messages from libraries) isn't a good idea.

desktop_version/src/FileSystemUtils.cpp

@@ -306,7 +306,7 @@ static int setLevelDirError(const char* text, ...)
     retval = SDL_vsnprintf(levelDirError, sizeof(levelDirError), text, list);
     va_end(list);
 
-    vlog_error(levelDirError);
+    vlog_error("%s", levelDirError);
 
     return retval;
 }

desktop_version/src/Graphics.cpp

@@ -321,7 +321,7 @@ void Graphics::updatetitlecolours(void)
         SDL_snprintf(error, sizeof(error), error_fmt, #tilesheet, tile_square); \
         SDL_snprintf(error_title, sizeof(error_title), error_title_fmt, #tilesheet); \
         \
-        vlog_error(error); \
+        vlog_error("%s", error); \
         \
         return false; \
     }