btl/sm: add missing #include <unistd.h>

Thanks Marco Atzeri for contributing the original patch

opal/mca/btl/sm/btl_sm.c

@@ -30,6 +30,9 @@
 
 #include <sys/types.h>
 #include <sys/stat.h>
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
 #ifdef HAVE_FCNTL_H
 #include <fcntl.h>
 #endif  /* HAVE_FCNTL_H */