Fix unused function warning in x509.c

Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
diff --git a/library/x509.c b/library/x509.c
index b600f45..a6335ca 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -134,6 +134,8 @@
 /*
  * Convert md type to string
  */
+#if !defined(MBEDTLS_X509_REMOVE_INFO) && defined(MBEDTLS_X509_RSASSA_PSS_SUPPORT)
+
 static inline const char *md_type_to_string(mbedtls_md_type_t md_alg)
 {
     switch (md_alg) {
@@ -172,6 +174,8 @@
     }
 }
 
+#endif
+
 #if defined(MBEDTLS_X509_RSASSA_PSS_SUPPORT)
 /*
  * HashAlgorithm ::= AlgorithmIdentifier