Autotools: Move libmount from Libs.private to Requires.private

libmount depends on libblkid, so if we statically link a trivial
executable to GIO, we need to pull in both libmount and libblkid.
Linking with -lmount is not enough to achieve that, but recursing
into mount.pc is.

This makes the Autotools build a bit more like the Meson build,
which already puts mount in Requires.private.

Signed-off-by: Simon McVittie <smcv@collabora.com>
11 jobs for <span class="ref-name">libmount</span> in 29 minutes and 14 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #112139
cross-android_api21_arm64

00:01:37

passed #112140
cross-android_api28_arm64

00:01:35

passed #112141
cross-mingw64

00:02:17

passed #112256
fedora-autotools-x86_64

00:05:41

passed #112137
fedora-x86_64

00:04:40

manual #112144
freebsd-11 allowed to fail manual
freebsd-11-x86_64
passed #112255
win32
msys2-mingw32

00:07:14

passed #112143
win32
vs2017-x64

00:07:34

failed #112138
fedora-autotools-x86_64

00:04:35

failed #112142
win32
msys2-mingw32

00:00:39

 
  Coverage
passed #112145
coverage

00:02:13

74.6%