Replace mbedtls_platform_enforce_volatile_reads 2

Replace remaining mbedtls_platform_enforce_volatile_reads() with
mbedtls_platform_random_delay().
diff --git a/library/pk.c b/library/pk.c
index 1e991c5..252c789 100644
--- a/library/pk.c
+++ b/library/pk.c
@@ -1553,7 +1553,7 @@
 
     if( verify_ret == 0 )
     {
-        mbedtls_platform_enforce_volatile_reads();
+        mbedtls_platform_random_delay(50);
         if( verify_ret == 0 )
         {
             return( verify_ret );