Avoid unused variable warning in X.509 CSR fuzzing

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/programs/fuzz/fuzz_x509csr.c b/programs/fuzz/fuzz_x509csr.c
index 9c21412..a270742 100644
--- a/programs/fuzz/fuzz_x509csr.c
+++ b/programs/fuzz/fuzz_x509csr.c
@@ -15,6 +15,7 @@
     }
 #else
     ((void) ret);
+    ((void) buf);
 #endif /* !MBEDTLS_X509_REMOVE_INFO */
     mbedtls_x509_csr_free( &csr );
 #else