build: Bump meson_version to 0.50
build: Bump meson_version to 0.50

To suppress the warning:

  WARNING: Project targetting '>= 0.48' but tried to use feature
  introduced in '0.50.0': install arg in configure_file

GNOME/libsecret,GNOME/libsecret,GNOME/libsecret,GNOME/libsecret