Puts platform time abstraction into its own header

Separates platform time abstraction into it's own header from the
general platform abstraction as both depend on different build options.
(MBEDTLS_PLATFORM_C vs MBEDTLS_HAVE_TIME)
diff --git a/library/x509.c b/library/x509.c
index a0df817..bc3bfe0 100644
--- a/library/x509.c
+++ b/library/x509.c
@@ -55,12 +55,15 @@
 #include <stdlib.h>
 #define mbedtls_free      free
 #define mbedtls_calloc    calloc
-#define mbedtls_time      time
-#define mbedtls_time_t    time_t
 #define mbedtls_printf    printf
 #define mbedtls_snprintf  snprintf
 #endif
 
+
+#if defined(MBEDTLS_HAVE_TIME)
+#include "mbedtls/platform_time.h"
+#endif
+
 #if defined(_WIN32) && !defined(EFIX64) && !defined(EFI32)
 #include <windows.h>
 #else