meson: Fix install-quiet after clean

libpq-oauth was missing from the installed_targets list, so

    $ ninja clean && ninja install-quiet

failed with the error message

    ERROR: File 'src/interfaces/libpq-oauth/libpq-oauth.a' could not be found

It seems a little odd to have to tell Meson what's missing, since it
clearly knows how to build that file during regular installation. But
the "quiet" variant we've created must use --no-rebuild, to avoid
spawning concurrent ninja processes that would step on each other.

Reported-by: Andres Freund <andres@anarazel.de>
Backpatch-through: 18
Discussion: https://postgr.es/m/hbpqdwxkfnqijaxzgdpvdtp57s7gwxa5d6sbxswovjrournlk6%404jnb2gzan4em

meson.build

@@ -3467,6 +3467,13 @@ installed_targets = [
   ecpg_targets,
 ]
 
+if oauth_flow_supported
+  installed_targets += [
+    libpq_oauth_so,
+    libpq_oauth_st,
+  ]
+endif
+
 # all targets that require building code
 all_built = [
   installed_targets,