Undefined reference error when trying to access a component using ECS [closed]

I'm trying to access the x and y values from PositionComponent in my MovementComponent but I get this error and I just can't figure out why I get this.

in my Entity class I've got a function GetComponent() that looks like this:

template<typename ComponentType>
ComponentType* Entity::GetComponent() {
    for(unsigned int i = 0; i < m_components.size(); i++) {
        if(ComponentType* cmp = dynamic_cast<ComponentType*>(m_components[i])) {
            return cmp;
        }
    }
    return NULL;
}

m_components is a vector of Component* that looks like this:

std::vector<Component*> m_components;

My MovementComponent looks like this, and I get an error for some reason here:

void MovementComponent::MoveRight() {
    m_ContainingEntity->GetComponent<PositionComponent>()->x++; //I get the error at this line.
}

Error: undefined reference to `PositionComponent* Entity::GetComponent()'

What did I do wrong? I've got a feeling that the solution is really simple and that I just didn't pay much attention.

Entity.h:

#ifndef __ENTITY_H__
#define __ENTITY_H__

#include <vector>
#include <cstddef>

class Component;
class ECSManager;

class Entity {
public:
    void AddComponent(Component* cmp);

    template <typename ComponentType>
    ComponentType* GetComponent();

private:
    friend class ECSManager;
    std::vector<Component*> m_components;
};

#endif // __ENTITY_H__

Entity.cpp:

#include "Entity.h"
#include "Component.h"

void Entity::AddComponent(Component* cmp) {
    m_components.push_back(cmp);
    cmp->m_ContainingEntity = this;
}

template<typename ComponentType>
ComponentType* Entity::GetComponent() {
    for(unsigned int i = 0; i < m_components.size(); i++) {
        if(ComponentType* cmp = dynamic_cast<ComponentType*>(m_components[i])) {
            return cmp;
        }
    }
    return NULL;
}

MovementComponent.h:

#ifndef __MOVEMENTCOMPONENT_H__
#define __MOVEMENTCOMPONENT_H__

#include "Component.h"

class MovementComponent: public Component {
public:
    void MoveRight();
    void MoveLeft();
private:
};

#endif // __MOVEMENTCOMPONENT_H__

MovementComponent.cpp:

#include "MovementComponent.h"
#include "Entity.h"
#include "PositionComponent.h"

void MovementComponent::MoveRight() {
    m_ContainingEntity->GetComponent<PositionComponent>()->x++;
}

void MovementComponent::MoveLeft() {

}