Swift pods cannot yet be integrated as static libraries

I am building a app by Flutter.I have got this error message when doing "pod install" and the pod install failed and stopped.

The error message:

[!] The following Swift pods cannot yet be integrated as static libraries:

The Swift pod DKPhotoGallery depends upon SDWebImage and SDWebImageFLPlugin, which do not define modules. To opt into those targets generating module maps (which is necessary to import them from Swift when building as static libraries), you may set use_modular_headers! globally in your Podfile, or specify :modular_headers => true for particular dependencies.

Where should I set "use_modular_headers!" in the Podfile? thanks!