qemu: explicitly enable docs so it errors out if i.e. python-Sphinx is broken

srcpkgs/qemu/template

@@ -1,7 +1,7 @@
 # Template file for 'qemu'
 pkgname=qemu
 version=4.1.0
-revision=2
+revision=3
 short_desc="Open Source Processor Emulator"
 maintainer="Helmut Pozimski <helmut@pozimski.eu>"
 license="GPL-2.0-or-later, LGPL-2.1-or-later"
@@ -61,7 +61,7 @@ do_configure() {
 		--disable-xen --enable-tpm \
 		--enable-vhost-net --enable-vnc-png --enable-virtfs \
 		--enable-libusb --disable-glusterfs --enable-snappy --enable-usb-redir \
-		--enable-pie --localstatedir=/var --datadir=/usr/lib \
+		--enable-pie --localstatedir=/var --datadir=/usr/lib --enable-docs \
 		$(vopt_enable virgl virglrenderer) $(vopt_enable opengl) $(vopt_enable spice) \
 		${want_sdl} \
 		$(vopt_enable smartcard) \