build-sys: Add missing header files to _SOURCES
build-sys: Add missing header files to _SOURCES

2 newly-added header files were not added to _SOURCES, breaking make
distcheck.

Acked-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>

SPICE/spice,fgouget/spice,ya-mouse/spice,SPICE/spice,SPICE/spice,fgouget/spice,fgouget/spice,fgouget/spice,ya-mouse/spice,ya-mouse/spice