fix: emscripten cmake issue (#5301)

Signed-off-by: Henry Schreiner <[email protected]>

Author: henryiii

tools/pybind11Common.cmake

@@ -83,7 +83,7 @@ if(CMAKE_SYSTEM_NAME MATCHES Emscripten AND NOT _pybind11_no_exceptions)
   if(CMAKE_VERSION VERSION_LESS 3.13)
     message(WARNING "CMake 3.13+ is required to build for Emscripten. Some flags will be missing")
   else()
-    if(_is_config)
+    if(is_config)
       set(_tmp_config_target pybind11::pybind11_headers)
     else()
       set(_tmp_config_target pybind11_headers)