Add missing erand48.c to libpq/.gitignore.

Oversight in commit 818fd4a6.  While at it, sync order of file list
in .gitignore with those in the Makefile.

src/interfaces/libpq/.gitignore

 /exports.list
-/base64.c
+/libpq.rc
+# .c files that are symlinked in from elsewhere
 /chklocale.c
 /crypt.c
+/erand48.c
 /getaddrinfo.c
 /getpeereid.c
 /inet_aton.c
 /inet_net_ntop.c
 /noblock.c
 /open.c
+/system.c
+/pgsleep.c
 /pg_strong_random.c
 /pgstrcasecmp.c
 /pqsignal.c
-/saslprep.c
-/scram-common.c
-/sha2.c
-/sha2_openssl.c
 /snprintf.c
 /strerror.c
 /strlcpy.c
-/system.c
 /thread.c
-/unicode_norm.c
 /win32error.c
 /win32setlocale.c
-/pgsleep.c
-/md5.c
 /ip.c
+/md5.c
+/base64.c
+/scram-common.c
+/sha2.c
+/sha2_openssl.c
+/saslprep.c
+/unicode_norm.c
 /encnames.c
 /wchar.c
-/libpq.rc