patch 8.0.1305: writefile() never calls fsync()

Problem:    Writefile() never calls fsync().
Solution:   Follow the 'fsync' option with override to enable or disable.
diff --git a/src/globals.h b/src/globals.h
index 3183632..ae62710 100644
--- a/src/globals.h
+++ b/src/globals.h
@@ -1449,6 +1449,9 @@
 #ifdef FEAT_LIBCALL
 EXTERN char_u e_libcall[]	INIT(= N_("E364: Library call failed for \"%s()\""));
 #endif
+#ifdef HAVE_FSYNC
+EXTERN char_u e_fsync[]		INIT(= N_("E667: Fsync failed"));
+#endif
 #if defined(DYNAMIC_PERL) \
 	|| defined(DYNAMIC_PYTHON) || defined(DYNAMIC_PYTHON3) \
 	|| defined(DYNAMIC_RUBY) \