Provide JunitReporter declaration with EXTERNAL_INTERFACES

Related to #991
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 3fb58ef..1921c2b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -248,6 +248,7 @@
         ${HEADER_DIR}/reporters/catch_reporter_automake.hpp
         ${HEADER_DIR}/reporters/catch_reporter_bases.hpp
         ${HEADER_DIR}/reporters/catch_reporter_compact.h
+        ${HEADER_DIR}/reporters/catch_reporter_junit.h
         ${HEADER_DIR}/reporters/catch_reporter_multi.h
         ${HEADER_DIR}/reporters/catch_reporter_tap.hpp
         ${HEADER_DIR}/reporters/catch_reporter_teamcity.hpp