Add zip_ftell.c to CMake build list.

Suggested by IohannRabeson on github.

lib/CMakeLists.txt

diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 1596f30..2cdc286 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -95,6 +95,7 @@
   zip_fopen_index_encrypted.c
   zip_fread.c
   zip_fseek.c
+  zip_ftell.c
   zip_get_archive_comment.c
   zip_get_archive_flag.c
   zip_get_compression_implementation.c