C header file problem

Question

test.h
int test(int x);

test.c
int test(intx){
return x;
}

int test2(){
return 10;
}

main.c
#include "main.h"

test2() ---> error not declared in scope

So these are not the actual files but this is the problem is there way to declare test2 without writing in test.h
Can i declare the function on top of the main.c kinda like

main.c
#include "main.h"
int test2();

test2()

Also i want it to retaim what is inside the function of course

Answer 1

~ swim ~ I dont own the header file in this case and not allowed to change it directly weird company rule i guess

Answer 2

~ swim ~ yes exactly

Answer 3

~ swim ~ wouldn't declaring top pf main.c work includes get replaced with the file anyway if i add it below #include test.h shouldnt it be like adding to it