[Bug c/81777] Compiler doesn't detect wrong function declaration

[jsiebert at poczta dot wp.pl]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=81777

--- Comment #1 from jsiebert at poczta dot wp.pl ---
file test.c:
#include "testarg.h"

int main(void)
{
        testarg();
        return 0;
}