Add missing source files to build.
Based on patch by Paul Harris.

lib/CMakeLists.txt

diff --git a/lib/CMakeLists.txt b/lib/CMakeLists.txt
index 2ea8f73..262074d 100644
--- a/lib/CMakeLists.txt
+++ b/lib/CMakeLists.txt
@@ -77,10 +77,12 @@
   zip_file_error_clear.c
   zip_file_error_get.c
   zip_file_get_comment.c
+  zip_file_get_external_attributes.c
   zip_file_get_offset.c
   zip_file_rename.c
   zip_file_replace.c
   zip_file_set_comment.c
+  zip_file_set_external_attributes.c
   zip_file_strerror.c
   zip_filerange_crc.c
   zip_fopen.c