[metadata] Remove warnings about missing prototypes

Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
[metadata] Remove warnings about unused functions/variables under Windows

Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>
[metadata] Fix warnings about prototypes,casting under Windows

Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>