Add example program for Curve25519

Getting a lot of questions about how to use it. This will hopefully get people
started.
diff --git a/programs/pkey/CMakeLists.txt b/programs/pkey/CMakeLists.txt
index 8dc7def..5a37a42 100644
--- a/programs/pkey/CMakeLists.txt
+++ b/programs/pkey/CMakeLists.txt
@@ -7,6 +7,9 @@
 add_executable(dh_server dh_server.c)
 target_link_libraries(dh_server mbedtls)
 
+add_executable(ecdh_curve25519 ecdh_curve25519.c)
+target_link_libraries(ecdh_curve25519 mbedtls)
+
 add_executable(ecdsa ecdsa.c)
 target_link_libraries(ecdsa mbedtls)