Fix the build when MBEDTLS_PLATFORM_C is unset

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/programs/test/dlopen.c b/programs/test/dlopen.c
index 2ee5316..62c8554 100644
--- a/programs/test/dlopen.c
+++ b/programs/test/dlopen.c
@@ -23,8 +23,9 @@
 #include MBEDTLS_CONFIG_FILE
 #endif
 
+#if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
-#if !defined(MBEDTLS_PLATFORM_C)
+#else
 #include <stdio.h>
 #include <stdlib.h>
 #define mbedtls_fprintf    fprintf