linux-waitpid: Get rid of format string warning
    
    Clang gives this warning:
    
    /home/emaisin/src/binutils-gdb/gdb/gdbserver/../nat/linux-waitpid.c:45:25: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
          vfprintf (stderr, format, args);
                            ^~~~~~
    
    Get rid of it by adding ATTRIBUTE_PRINTF.
    
    gdb/ChangeLog:
    
    	* nat/linux-waitpid.c (linux_debug): Add ATTRIBUTE_PRINTF.