gitignore: Support symbolic link for target directory

Author: uklotzde

.gitignore

@@ -1,6 +1,8 @@
 # Generated by Cargo
-# will have compiled files and executables
-**/target/
+# - will have compiled files and executables
+# - without a trailing slash to allow using a symbolic link
+#   for out-of-tree builds on a separate volume
+**/target
 
 # These are backup files generated by rustfmt
 **/*.rs.bk