lcdproc: use ac_cv_mtab_file

Set ac_cv_mtab_file to /etc/mtab otherwise build will fail if no mtab,
mnttab or fstab is found in /etc (on host)

Fixes:
 - http://autobuild.buildroot.org/results/efaf2833d674c7e366c59f367f0b83c7f88546bb

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/lcdproc/lcdproc.mk

@@ -18,7 +18,9 @@ endif
 
 LCDPROC_DEPENDENCIES = freetype ncurses zlib
 
-LCDPROC_CONF_ENV += ac_cv_path_FT2_CONFIG=$(STAGING_DIR)/usr/bin/freetype-config
+LCDPROC_CONF_ENV += \
+	ac_cv_mtab_file=/etc/mtab \
+	ac_cv_path_FT2_CONFIG=$(STAGING_DIR)/usr/bin/freetype-config
 
 ifeq ($(BR2_PACKAGE_LIBPNG),y)
 LCDPROC_DEPENDENCIES += libpng