Provide XmlReporter declaration with EXTERNAL_INTERFACES

Related to #991
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1921c2b..ad96fd3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -252,6 +252,7 @@
         ${HEADER_DIR}/reporters/catch_reporter_multi.h
         ${HEADER_DIR}/reporters/catch_reporter_tap.hpp
         ${HEADER_DIR}/reporters/catch_reporter_teamcity.hpp
+        ${HEADER_DIR}/reporters/catch_reporter_xml.h
         )
 set(REPORTER_SOURCES
         ${HEADER_DIR}/reporters/catch_reporter_bases.cpp