libsecret: upgrade 0.20.1 -> 0.20.3 / port to meson

libsecret: upgrade 0.20.1 -> 0.20.3 / port to meson

>From [1]:

0.20.3
 * secret-file-backend: Fix use-after-free in flatpak [!52]
 * docs: Add man subdir only if manpage is enabled [!51]

>From [2]:

0.20.2
 * secret-file-collection: force little-endian in GVariant [!49, #42]
 * Prefer g_info() over g_message() [!48, #40]
 * meson: Don't specify shared_library() [!47]
 * docs: Make sure to set install: true [!46]

[1] http://ftp.gnome.org/pub/gnome/sources/libsecret/0.20/libsecret-0.20.3.news
[2] http://ftp.gnome.org/pub/gnome/sources/libsecret/0.20/libsecret-0.20.2.news

Signed-off-by: Andreas Müller <schnitzeltony@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>