Fix missing #includes in system/core.

Change-Id: I4736e8c4f7bb8bb03640d26d58243efaa67dc75c
diff --git a/fastbootd/utils.c b/fastbootd/utils.c
index bef2463..a9ab47e 100644
--- a/fastbootd/utils.c
+++ b/fastbootd/utils.c
@@ -28,6 +28,7 @@
  * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  * SUCH DAMAGE.
  */
+#include <errno.h>
 #include <sys/types.h>
 #include <sys/stat.h>
 #include <unistd.h>