logger: fix compilation on OS/2

logger: fix compilation on OS/2

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