Add missing .gitignore entry.

Seems an oversight in 7d8219a.  Fix before somebody commits
a generated file.

Author: tglsfdc

src/bin/pg_waldump/.gitignore

@@ -15,6 +15,7 @@
 /nbtdesc.c
 /relmapdesc.c
 /replorigindesc.c
+/rmgrdesc_utils.c
 /seqdesc.c
 /smgrdesc.c
 /spgdesc.c