Assorted header fixes

Playing around with some g-i related stuff and hit missing symbols

Clutter

• clutter_input_device_get_n_axes
• clutter_input_device_get_axis
• clutter_input_device_get_axis_value

These seem to have been removed at some point, yet the declarations stayed (and so are present in GIR)

Meta:

• meta_backend_set_numlock

Same as Clutter

Cogl:

• cogl_blend_string_error_quark

Does actually exist but the symbol wasn't exported