A static variable inside a class is actually an external declaration. You still need the variable definition. This is similar to C external variables.
So in the .h file:
class Bullet
{
public:
static bool active;
};
and in the .cpp file, at global scope:
bool Bullet::active = false;
The lack of the variable definition (not declaration) is deduced because your error message actually comes from the linker, not the compiler.