Add includedir for Gentoo
Add includedir for Gentoo

This breaks otherwise for setups using STRICT_VARIABLES=yes

puppetlabs/puppetlabs-mysql,markasammut/puppetlabs-mysql,markasammut/puppetlabs-mysql,simp/puppetlabs-mysql,simp/puppetlabs-mysql,DavidS/puppetlabs-mysql,DavidS/puppetlabs-mysql,puppetlabs/puppetlabs-mysql