Include what we use.

Test: mma
Bug: None
Change-Id: I259468e829052bf27fab599ff951a5a2638d35aa
Exempt-From-Owner-Approval: trivial cleanup
diff --git a/libc/system_properties/prop_area.cpp b/libc/system_properties/prop_area.cpp
index e11f292..032fa4e 100644
--- a/libc/system_properties/prop_area.cpp
+++ b/libc/system_properties/prop_area.cpp
@@ -26,6 +26,7 @@
  * SUCH DAMAGE.
  */
 
+#include <errno.h>
 #include <fcntl.h>
 #include <stdlib.h>
 #include <sys/cdefs.h>