Add missing dependencies on certs.c in fuzz programs

Fix `make programs` when MBEDTLS_CERTS_C is disabled.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/programs/fuzz/fuzz_server.c b/programs/fuzz/fuzz_server.c
index 5f970e9..e161d7e 100644
--- a/programs/fuzz/fuzz_server.c
+++ b/programs/fuzz/fuzz_server.c
@@ -55,7 +55,8 @@
     options = Data[Size - 1];
 
     if (initialized == 0) {
-#if defined(MBEDTLS_X509_CRT_PARSE_C) && defined(MBEDTLS_PEM_PARSE_C)
+#if defined(MBEDTLS_X509_CRT_PARSE_C) && defined(MBEDTLS_PEM_PARSE_C) && \
+        defined(MBEDTLS_CERTS_C)
         mbedtls_x509_crt_init(&srvcert);
         mbedtls_pk_init(&pkey);
         if (mbedtls_x509_crt_parse(&srvcert, (const unsigned char *) mbedtls_test_srv_crt,