Try this,

Add the following line in your add.h file. Like,

#ifndef ADD1_H
#define ADD1_H
#include "add.c"

int addnum(int a, int b);

Other wise,

If you have an individual C function that you want to call, and for some reason you don't have or don't want to #include a C header file in which that function is declared, you can declare the individual C function in your C++ code using the extern "C" syntax. You need to use the full function prototype:

   extern "C" void f(int i, char c, );
 extern "C" {
   void   f(int i, char c);
   int    g(char* s, char* s2);
Call the function like,

int main()
   f(10, 'x');