docs: Fix standalone INSTALL, broken in 06c70849fb

We should probably check that INSTALL can be generated in CI.

Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/795075.1700254192@sss.pgh.pa.us

doc/src/sgml/standalone-profile.xsl

@@ -90,4 +90,8 @@ variant without links and references to the main documentation.
   <xsl:text>xml2</xsl:text>
 </xsl:template>
 
+<xsl:template match="xref[@linkend='sepgsql']">
+  <xsl:text>sepgsql</xsl:text>
+</xsl:template>
+
 </xsl:stylesheet>