Avoid use of wildcard in pg_waldump's .gitignore.

This would be all right, maybe, if it didn't also match a file that
definitely should not be ignored.  We don't add rmgrs so often that
manual maintenance of this file list is impractical, so just write
out the list.

(I find the equivalent wildcard use in the Makefile pretty lazy and
unsafe as well, but will leave that alone until it actually causes a
problem.)

Per bug #16042 from Denis Stuchalin.

Discussion: https://postgr.es/m/16042-c174ee692ac21cbd@postgresql.org

src/bin/pg_waldump/.gitignore

 /pg_waldump
+
 # Source files copied from src/backend/access/rmgrdesc/
-/*desc.c
+/brindesc.c
+/clogdesc.c
+/committsdesc.c
+/dbasedesc.c
+/genericdesc.c
+/gindesc.c
+/gistdesc.c
+/hashdesc.c
+/heapdesc.c
+/logicalmsgdesc.c
+/mxactdesc.c
+/nbtdesc.c
+/relmapdesc.c
+/replorigindesc.c
+/seqdesc.c
+/smgrdesc.c
+/spgdesc.c
+/standbydesc.c
+/tblspcdesc.c
+/xactdesc.c
+/xlogdesc.c
 /xlogreader.c
 
 # Generated by test suite