emulators/qemu: Fix build without DOCS

• While here also add missing X11 dependencies to the GTK2 option

/usr/bin/install -c -c -m 0644 docs/qemu-qmp-ref.7 "/wrkdirs/usr/ports/emulators/qemu/work/stage/usr/local/man/man7"
install: docs/qemu-qmp-ref.7: No such file or directory
gmake[1]: *** [Makefile:576: install-doc] Error 71
gmake[1]: Leaving directory '/wrkdirs/usr/ports/emulators/qemu/work/qemu-2.9.0'

PR: 221749
Submitted by: smh
Reviewed by: novel
Approved by: bofh
MFH: 2017Q4