Include what you use: <unistd.h>

This was transitively included, but no longer is, so it must be
explicitly included.

Test: build
Bug: 157822244
Change-Id: Ifa85582c9380df6f16025eba86b64e9af32bc226
Merged-In: Ifa85582c9380df6f16025eba86b64e9af32bc226
diff --git a/server/PppController.cpp b/server/PppController.cpp
index 80a36e9..b80e1a6 100644
--- a/server/PppController.cpp
+++ b/server/PppController.cpp
@@ -14,10 +14,11 @@
  * limitations under the License.
  */
 
-#include <stdlib.h>
 #include <errno.h>
 #include <fcntl.h>
+#include <stdlib.h>
 #include <string.h>
+#include <unistd.h>
 
 #include <sys/socket.h>
 #include <sys/stat.h>