Fix cmake run on Ubuntu 16.04 (and possibly other distros)

Apparently that platform trips over a missing newline at the end of the generated POTFILES.in.in file.
Adding one allows cmake to complete configuration and appears to be fine on Fedora as well
(although it worked fine without the newline too).

Thanks go to Michael Jung <gnucash-devel@mailinglists.mjung.org> for reporting this and proposing the fix.

po/CMakeLists.txt

@@ -105,6 +105,7 @@ FUNCTION(MAKE_GNUCASH_POTFILES)
   # here so it is easier for me to compare to the autotools
   # generated POTFILES.in
   STRING(REPLACE ";" "\n" SORT_IN "${FILES}")
+  STRING(APPEND SORT_IN "\n")
   FILE(WRITE ${CMAKE_CURRENT_BINARY_DIR}/POTFILES.in.in "${SORT_IN}")
 
   EXECUTE_PROCESS(COMMAND "${PERL_EXECUTABLE}" "${CMAKE_SOURCE_DIR}/util/elegant-sort.pl"