Add missing bracket

Wasn't spotted earlier because it's guarded by `! HAVE_GETRANDOM`.
diff --git a/library/entropy_poll.c b/library/entropy_poll.c
index ae3d2d5..4556f88 100644
--- a/library/entropy_poll.c
+++ b/library/entropy_poll.c
@@ -135,7 +135,7 @@
         return( MBEDTLS_ERR_ENTROPY_SOURCE_FAILED );
     /* Fall through if the system call isn't known. */
 #else
-    ((void) ret;
+    ((void) ret);
 #endif /* HAVE_GETRANDOM */
 
     *olen = 0;