fastboot: add errno.h to fix windows build

The removal of extra headers from ext4_utils.h removed an indirect
include of errno.h that breaks building with USE_MINGW=1.

Change-Id: Ic8448db97b5c056a139dc836e1d6f3aaafb917c9
diff --git a/fastboot/engine.c b/fastboot/engine.c
index 6c636cd..8d46991 100644
--- a/fastboot/engine.c
+++ b/fastboot/engine.c
@@ -29,6 +29,7 @@
 #include "fastboot.h"
 #include "make_ext4fs.h"
 
+#include <errno.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <stdarg.h>