Upgrade mtools to 3.0.37

This version contains a fix for -Wstring-concatenation error.

Test: presubmit
Bug: 175068488
Change-Id: Id4f2ae49da91a481ab2625fdf17668689b640794
diff --git a/privileges.c b/privileges.c
index 71de4c1..b055003 100644
--- a/privileges.c
+++ b/privileges.c
@@ -200,7 +200,7 @@
 #endif
 #endif
 	}
-	
+
 	drop_privs();
 	print_privs("after init, real should be 0, effective should not ");
 }