To unlock all skills you must do a number of things:
You must reach level 50. (You should get this at around the end of the story if you have been evading police scans and chases as well).
You must complete all 15 gang hideouts.
This will give you the required amount of skill points to unlock all skills. In addition there are 5 locked skills, the requirements of which are:
Complete 1 Criminal Convoy to unlock the 'Critical Focus' skill. (Thanks AnimaL EquinoX).
Complete 5 fixer missions to unlock the 'Offensive driver' skill.
Complete 5 gang hideouts for the 'Rapid reload' skill.
Complete 10 chess games for the 'Maximized focus' skill
Complete wave 5 in an NVZN phone game for the 'Auto Weapons Expert' skill.
For the Chess games it is cumulative, so you don't have to complete all 10 games at 1 table, you can do it over a number of tables. You can find chess games all over the city.
The NVZN phone games is a blue marker on your map under 'AR Phone Game'. There are about 5 to choose from, all over the city as well.
For some reason you have 1 skill point left over, i've put it down to a miscalculation on the devs part.