Use correct include for 'write()' on POSIX systems

Fixes compilation errors in ctkErrorLogModel.cpp for gcc 4.7
and clang.

Libs/Core/ctkErrorLogModel.cpp

@@ -39,6 +39,8 @@
 #include <cstdio> // For _fileno or fileno
 #ifdef _MSC_VER
 # include <io.h> // For _write()
+#else
+# include <unistd.h>
 #endif
 
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