cmake: link programs that only use x509 with libmbedx509

When building with CMake, for sample programs that only use
functionality in libmbedcrypto and libmbedx509, link with
libmbedx509, not with libmbedtls.

cert_app makes a TLS connection, so do link it with libmbedtls.
diff --git a/programs/x509/CMakeLists.txt b/programs/x509/CMakeLists.txt
index 39b8b5b..68dec99 100644
--- a/programs/x509/CMakeLists.txt
+++ b/programs/x509/CMakeLists.txt
@@ -1,5 +1,5 @@
 set(libs
-    mbedtls
+    mbedx509
 )
 
 if(USE_PKCS11_HELPER_LIBRARY)
@@ -11,7 +11,7 @@
 endif(ENABLE_ZLIB_SUPPORT)
 
 add_executable(cert_app cert_app.c)
-target_link_libraries(cert_app ${libs})
+target_link_libraries(cert_app ${libs} mbedtls)
 
 add_executable(crl_app crl_app.c)
 target_link_libraries(crl_app ${libs})