pmix/base: add missing #include <unistd.h>

Thanks Marco Atzeri for contributing the original patch

opal/mca/pmix/base/pmix_base_fns.c

@@ -21,6 +21,9 @@
 
 #include <time.h>
 #include <string.h>
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
 
 #include "opal_stdint.h"
 #include "opal/class/opal_pointer_array.h"