Makefile: Retain XML Declaration and DTD in docs.

Since Asterisk 12, the documentation got an XML Stylesheet. Because of a typo,
the XML Declaration and DTD were overwritten by this.

ASTERISK-26212 #close

Change-Id: If5ee4625068042e98ab3fcb22a25e2f15d0c68bd

Makefile

@@ -491,7 +491,7 @@ doc/core-en_US.xml: makeopts .lastclean $(XML_core_en_US)
 	@printf "Building Documentation For: "
 	@echo "<?xml version=\"1.0\" encoding=\"UTF-8\"?>" > $@
 	@echo "<!DOCTYPE docs SYSTEM \"appdocsxml.dtd\">" >> $@
-	@echo "<?xml-stylesheet type=\"text/xsl\" href=\"appdocsxml.xslt\"?>" > $@
+	@echo "<?xml-stylesheet type=\"text/xsl\" href=\"appdocsxml.xslt\"?>" >> $@
 	@echo "<docs xmlns:xi=\"http://www.w3.org/2001/XInclude\">" >> $@
 	@for x in $(MOD_SUBDIRS); do \
 		printf "$$x " ; \
@@ -515,7 +515,7 @@ else
 	@printf "Building Documentation For: "
 	@echo "<?xml version=\"1.0\" encoding=\"UTF-8\"?>" > $@
 	@echo "<!DOCTYPE docs SYSTEM \"appdocsxml.dtd\">" >> $@
-	@echo "<?xml-stylesheet type=\"text/xsl\" href=\"appdocsxml.xslt\"?>" > $@
+	@echo "<?xml-stylesheet type=\"text/xsl\" href=\"appdocsxml.xslt\"?>" >> $@
 	@echo "<docs xmlns:xi=\"http://www.w3.org/2001/XInclude\">" >> $@
 	@for x in $(MOD_SUBDIRS); do \
 		printf "$$x " ; \