logger: fix compilation on OS/2

```--
  CC       logger/console.lo
logger/console.c: In function 'Open':
logger/console.c:115:5: error: implicit declaration of function 'isatty' [-Werror=implicit-function-declaration]
     if (isatty(STDERR_FILENO) && var_InheritBool(obj, "color"))
     ^
logger/console.c:115:16: error: 'STDERR_FILENO' undeclared (first use in this function)
     if (isatty(STDERR_FILENO) && var_InheritBool(obj, "color"))
                ^
logger/console.c:115:16: note: each undeclared identifier is reported only once for each function it appears in
cc1.exe: some warnings being treated as errors
```

--
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/logger/console.c

@@ -26,6 +26,7 @@
 #include <stdarg.h>
 #include <stdio.h>
 #include <inttypes.h>
+#include <unistd.h>         /* isatty(), STDERR_FILNO */
 
 #include <vlc_common.h>
 #include <vlc_plugin.h>