sdk: The main in the C example should return an int

see C17 (ISO/IEC 9899:2018)

(From yocto-docs rev: 1ebaed299f7ef80a49b68608f45bf25884900d13)

Signed-off-by: Jan Vermaete <jan.vermaete@gmail.com>
Signed-off-by: Antonin Godard <antonin.godard@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

documentation/sdk-manual/working-projects.rst

@@ -56,9 +56,10 @@ project:
 
          #include <stdio.h>
 
-         main()
+         int main()
              {
                  printf("Hello World!\n");
+                 return 0;
              }
 
    -  ``configure.ac``::