pkcs7: build under CMake

The patch updates CMakeLists.txt to include pkcs7.

Signed-off-by: Daniel Axtens <dja@axtens.net>
diff --git a/library/CMakeLists.txt b/library/CMakeLists.txt
index 378cfb4..aed4a05 100644
--- a/library/CMakeLists.txt
+++ b/library/CMakeLists.txt
@@ -89,6 +89,7 @@
 )
 
 set(src_x509
+    pkcs7.c
     x509.c
     x509_create.c
     x509_crl.c