Fix missing #includes in system/core.

Change-Id: I4736e8c4f7bb8bb03640d26d58243efaa67dc75c
diff --git a/fastbootd/commands/partitions.c b/fastbootd/commands/partitions.c
index f2c9da7..3b27959 100644
--- a/fastbootd/commands/partitions.c
+++ b/fastbootd/commands/partitions.c
@@ -31,6 +31,7 @@
 
 #include <sys/types.h>
 #include <sys/stat.h>
+#include <errno.h>
 #include <fcntl.h>
 #include <sys/mman.h>
 #include <sys/stat.h>