include: cleanup for -Wsystem-header

- warnings as errors, or errors introduced when -Wsystem-header
  was experimentally introduced.

Change-Id: Ia8f5b3c1c1096e191741223d52526aa48c7f1cc4
diff --git a/include/cutils/fs.h b/include/cutils/fs.h
index d1d4cf2..70f0b92 100644
--- a/include/cutils/fs.h
+++ b/include/cutils/fs.h
@@ -18,6 +18,7 @@
 #define __CUTILS_FS_H
 
 #include <sys/types.h>
+#include <unistd.h>
 
 /*
  * TEMP_FAILURE_RETRY is defined by some, but not all, versions of