xproto_inputproto: also disable asciidoc documentation for host builds

Fixes:
http://autobuild.buildroot.net/results/0ff/0ff9a3a27984ad91aa33079143bb87ed71bfe7c4/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/x11r7/xproto_inputproto/xproto_inputproto.mk

@@ -12,6 +12,7 @@ XPROTO_INPUTPROTO_LICENSE_FILES = COPYING
 XPROTO_INPUTPROTO_INSTALL_STAGING = YES
 XPROTO_INPUTPROTO_INSTALL_TARGET = NO
 XPROTO_INPUTPROTO_CONF_ENV = ac_cv_path_ASCIIDOC=""
+HOST_XPROTO_INPUTPROTO_CONF_ENV = ac_cv_path_ASCIIDOC=""
 
 $(eval $(autotools-package))
 $(eval $(host-autotools-package))