docbook: fix rm'ed files mentioned in .d build error

When some .xml include file is deleted or removed, but the .d
already exists, the build will fail trying to build the apparently
missing file, requiring a make proper.
This fix enclosed the prerequisites in $(wildcard ...), when
generating the .d file.

docbook/dep.xsl

@@ -8,8 +8,9 @@
 	<xsl:template match="/">
 		<xsl:document href="{$output}" method="text" indent="no" 
 					  omit-xml-declaration="yes">
-			<xsl:value-of select="concat($output, ': ')"/>
+			<xsl:value-of select="concat($output, ':$(wildcard  ')"/>
 			<xsl:apply-templates mode="subroot"/>
+			<xsl:text>)&#xA;</xsl:text>
 		</xsl:document>
 	</xsl:template>