We are generating Yocto SDK using the following command: bitbake -c populate_sdk <image-name>
Yocto Branch : Dunfell
We don't see header files getting included in the SDK, for example we have libmodbus part of IMAGE_INSTALL, but we don't see the modbus.h file (which is present in libmodbus-dev package) in SDK.
Could you execute the command below so we can verify your setup:
bitbake -e <image-name> | grep SDKIMAGE_FEATURES
Development packages are automatically included into SDK when they are installed into the image when SDKIMAGE_FEATURES variable defines it
SDKIMAGE_FEATURES = "dev-pkgs staticdev-pkgs"
https://www.yoctoproject.org/docs/3.1/mega-manual/mega-manual.html#var-SDKIMAGE_FEATURES