Made the header-file filter test more explicit
Made the header-file filter test more explicit

Previously we were applying it if any "show" option had been provided at
all, here we make sure it is spelt correctly to ensure good behaviour.

kirbyfan64/breathe,kirbyfan64/breathe,RR2DO2/breathe,RR2DO2/breathe,AnthonyTruchet/breathe,kirbyfan64/breathe,AnthonyTruchet/breathe,kirbyfan64/breathe,AnthonyTruchet/breathe,AnthonyTruchet/breathe