pkey Cmakelists: Updated the set libs to be consistent with others.

Signed-off-by: Minos Galanakis <minos.galanakis@arm.com>
diff --git a/programs/pkey/CMakeLists.txt b/programs/pkey/CMakeLists.txt
index c7ebcb0..b747a3f 100644
--- a/programs/pkey/CMakeLists.txt
+++ b/programs/pkey/CMakeLists.txt
@@ -1,5 +1,5 @@
 set(libs
-    mbedtls
+    ${mbedtls_target}
 )
 
 set(executables_mbedtls