rustfmt: Drop lightning-invoice/tests/ser_de.rs from exclusion list

Author: tnull

rustfmt_excluded_files

@@ -5,7 +5,6 @@
 ./lightning-invoice/src/ser.rs
 ./lightning-invoice/src/tb.rs
 ./lightning-invoice/src/utils.rs
-./lightning-invoice/tests/ser_de.rs
 ./lightning-persister/src/fs_store.rs
 ./lightning-persister/src/lib.rs
 ./lightning-persister/src/test_utils.rs