Use the normal idiom to support MBEDTLS_CONFIG_FILE

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/programs/test/dlopen.c b/programs/test/dlopen.c
index e31cae1..2ee5316 100644
--- a/programs/test/dlopen.c
+++ b/programs/test/dlopen.c
@@ -17,7 +17,11 @@
  *  limitations under the License.
  */
 
+#if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"
+#else
+#include MBEDTLS_CONFIG_FILE
+#endif
 
 #include "mbedtls/platform.h"
 #if !defined(MBEDTLS_PLATFORM_C)