gschema.dtd: Add target attribute to alias

The implementation requires it, but the DTD doesn’t even mention it.
11 jobs for wip/ernestask/dtd-fix in 28 minutes and 24 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  Build
passed #292304
cross-android_api21_arm64

00:01:55

passed #292305
cross-android_api28_arm64

00:01:55

passed #292306
cross-mingw64

00:02:26

passed #292302
debian-stable-x86_64

00:02:46

passed #293196
fedora-x86_64

00:04:41

passed #292309
freebsd-11
freebsd-11-x86_64

00:04:03

passed #292303
G_DISABLE_ASSERT

00:02:47

passed #292307
win32
msys2-mingw32

00:09:33

passed #292308
win32
vs2017-x64

00:08:52

failed #292301
fedora-x86_64

00:05:31

 
  Coverage
passed #292310
coverage

00:01:56

75.2%