Invert gate on printf attribute

Inverted the logic without thinking.

Signed-off-by: Paul Elliott <paul.elliott@arm.com>
diff --git a/include/mbedtls/debug.h b/include/mbedtls/debug.h
index ff8aec0..168f70d 100644
--- a/include/mbedtls/debug.h
+++ b/include/mbedtls/debug.h
@@ -93,10 +93,10 @@
  * This module provides debugging functions.
  */
 #if defined(__GNUC__)
-#define MBEDTLS_PRINTF_ATTRIBUTE(string_index, first_to_check)
-#else
 #define MBEDTLS_PRINTF_ATTRIBUTE(string_index, first_to_check)    \
     __attribute__((format (printf, string_index, first_to_check)))
+#else
+#define MBEDTLS_PRINTF_ATTRIBUTE(string_index, first_to_check)
 #endif
 
 #ifdef __cplusplus