srv: avoid a build warning on non-Linux systems

Due to an ifdef __linux__ we can have an unused variable "tio" on other
systems that produces this warning:

    CC      src/bt.o
  src/bt.c:851:17: warning: unused variable 'tio' [-Wunused-variable]
          struct termios tio;
                         ^
  1 warning generated.
    LD      bin/bt

Let's enclose it in the same ifdef.

Author: wtarreau

src/bt.c

@@ -848,7 +848,9 @@ static int serial_port_cmp(const void *a, const void *b)
  */
 int file_isatty(const char *devname)
 {
+#ifdef __linux__
 	struct termios tio;
+#endif
 	struct winsize ws;
 	struct stat st;
 	int ret = 0;