configure.ac: Only check for xinerama for client builds
configure.ac: Only check for xinerama for client builds

Xinerama support is only used for the X11 client, but is currently
being checked even for server only builds. This commit ensures Xinerama
is not checked for/added to spice-server.pc when not building the
client.

ya-mouse/spice,Fantu/spice,SPICE/spice,freedesktop-unofficial-mirror/spice__spice,SPICE/spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,Fantu/spice,freedesktop-unofficial-mirror/spice__spice,ya-mouse/spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,Fantu/spice,Fantu/spice,ya-mouse/spice,freedesktop-unofficial-mirror/spice__spice,SPICE/spice,fgouget/spice,fgouget/spice