meson: Don't specify shared_library()

Some people might want to statically link to libsecret, so use the more
general `library()` target to build libsecret.
6 jobs for wip/nielsdg/dont-specify-library-type in 9 minutes and 45 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #555258
fedora:asan

00:06:34

passed #555260
allowed to fail
fedora:coverage

00:05:37

69.8%
passed #555259
fedora:ubsan

00:07:10

passed #555257
fedora:Werror

00:09:10

 
  Deploy
passed #555261
allowed to fail
pages

00:00:35

passed pages:deploy

00:00:00