Add missing header file which is needed to build some plugins.

Author: Jan Vcelak <jvcelak@redhat.com>

diff a/CMakeLists.txt b/CMakeLists.txt
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -601,6 +601,7 @@ set(fatrat_DEV_HEADERS_ENGINES
 	${CMAKE_CURRENT_SOURCE_DIR}/src/engines/CurlPoller.h
 	${CMAKE_CURRENT_SOURCE_DIR}/src/engines/CurlStat.h
 	${CMAKE_CURRENT_SOURCE_DIR}/src/engines/UrlClient.h
+	${CMAKE_CURRENT_SOURCE_DIR}/src/engines/StaticTransferMessage.h
 	)
 
 install(FILES