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/include/mbedtls/ssl.h b/include/mbedtls/ssl.h
index bc62336..c0bfd3e 100644
--- a/include/mbedtls/ssl.h
+++ b/include/mbedtls/ssl.h
@@ -52,7 +52,7 @@
 #endif
 
 #if defined(MBEDTLS_HAVE_TIME)
-#include "platform.h"
+#include "mbedtls/platform_time.h"
 #endif
 
 /*