alsa-card-profiles: simplify adding custom profiles

It is sometimes useful to add a custom profile-set.
For that, e.g. `default.conf` needs to be modified.
(at least, i have not succeeded in just adding a new file)

But that change gets overridden when the package is updated,
which could be *extremely* dangerous, e.g. if said profile
changed the `volume-limit`.

By shipping an "empty" `9999-custom.conf`,
the update becomes less problematic,
because one can now use e.g. `dpkg-divert` on said file.

Refs. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1050293

spa/plugins/alsa/mixer/profile-sets/9999-custom.conf

@@ -0,0 +1,22 @@
+# This file is part of PulseAudio.
+#
+# PulseAudio is free software; you can redistribute it and/or modify
+# it under the terms of the GNU Lesser General Public License as
+# published by the Free Software Foundation; either version 2.1 of the
+# License, or (at your option) any later version.
+#
+# PulseAudio is distributed in the hope that it will be useful, but
+# WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+# General Public License for more details.
+#
+# You should have received a copy of the GNU Lesser General Public License
+# along with PulseAudio; if not, see <http://www.gnu.org/licenses/>.
+
+; Put your custom profiles here.
+
+; An example for defining multiple-sink profiles
+#[Profile output:analog-stereo+output:iec958-stereo+input:analog-stereo]
+#description = Foobar
+#output-mappings = analog-stereo iec958-stereo
+#input-mappings = analog-stereo

spa/plugins/alsa/mixer/profile-sets/default.conf

@@ -573,8 +573,4 @@ fallback = yes
 priority = 1
 direction = input
 
-; An example for defining multiple-sink profiles
+.include 9999-custom.conf
-#[Profile output:analog-stereo+output:iec958-stereo+input:analog-stereo]
-#description = Foobar
-#output-mappings = analog-stereo iec958-stereo
-#input-mappings = analog-stereo