feat(iOS): enable AVMedia

Previously, AVMedia was disabled entirely for iOS. As we intend to
support AVMedia in Collabora Office Mobile for iOS, this is rather
inconvenient. In reality, we can support some of the AVMedia stuff -
albeit using webkit to play videos rather than gstreamer.

Change-Id: I1c5b791f7f37185c2b92ba5e6d41f934ac215411
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/184333
Tested-by: Jenkins
Reviewed-by: Miklos Vajna <vmiklos@collabora.com>

distro-configs/LibreOfficeiOS.conf

@@ -13,11 +13,11 @@
 --with-macosx-version-min-required=10.11
 
 --disable-avahi
---disable-avmedia
 --disable-compiler-plugins
 --disable-cups
 --disable-dconf
 --disable-extensions
+--disable-gstreamer-1-0
 --disable-kf5
 --disable-odk
 --disable-openssl