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/ssl_cli.c b/library/ssl_cli.c
index cd39db0..358dc467 100644
--- a/library/ssl_cli.c
+++ b/library/ssl_cli.c
@@ -33,8 +33,6 @@
 #include <stdlib.h>
 #define mbedtls_calloc    calloc
 #define mbedtls_free      free
-#define mbedtls_time      time
-#define mbedtls_time_t    time_t
 #endif
 
 #include "mbedtls/debug.h"
@@ -46,7 +44,7 @@
 #include <stdint.h>
 
 #if defined(MBEDTLS_HAVE_TIME)
-#include <time.h>
+#include "mbedtls/platform_time.h"
 #endif
 
 #if defined(MBEDTLS_SSL_SESSION_TICKETS)