Add x509 library to utility programs

This commit adds Mbed TLS x509 as a library dependency for utility
programs. This is necessary for strerror.c to work correctly.

Signed-off-by: Harry Ramsey <harry.ramsey@arm.com>
diff --git a/programs/util/CMakeLists.txt b/programs/util/CMakeLists.txt
index 264d941..9ceb13f 100644
--- a/programs/util/CMakeLists.txt
+++ b/programs/util/CMakeLists.txt
@@ -1,5 +1,6 @@
 set(libs
     ${mbedcrypto_target}
+    ${mbedx509_target}
 )
 
 set(executables