The watchdogd package in Buildroot is missing the `INSTALL_STAGING = YES` option in it's makefile. The package provides a library called `libwdog.so`, which is linked against when process supervision is used. Without installing in the staging directory, programs that link against libwdog cannot find the pkgconfig file when configuring and fail to build. Adding `INSTALL_STAGING = YES` to watchdogd.mk fixes the build for programs that use libwdog.
Fixed by https://git.buildroot.org/buildroot/commit/?id=faeebe0858f40d18d7527b7d1de6a6868004c9e6, thanks