Adds missing conditions for platform time

In platform.c, made the time functions dependent on the configuration
MBEDTLS_HAVE_TIME to fix a build break where the functions could be
built but the mbedtls_time_t was not defined.
diff --git a/library/platform.c b/library/platform.c
index 68ca45d..2591c45 100644
--- a/library/platform.c
+++ b/library/platform.c
@@ -190,6 +190,8 @@
 }
 #endif /* MBEDTLS_PLATFORM_EXIT_ALT */
 
+#if defined(MBEDTLS_HAVE_TIME)
+
 #if defined(MBEDTLS_PLATFORM_TIME_ALT)
 #if !defined(MBEDTLS_PLATFORM_STD_TIME)
 /*
@@ -213,6 +215,8 @@
 }
 #endif /* MBEDTLS_PLATFORM_TIME_ALT */
 
+#endif /* MBEDTLS_HAVE_TIME */
+
 #if defined(MBEDTLS_ENTROPY_NV_SEED)
 #if !defined(MBEDTLS_PLATFORM_NO_STD_FUNCTIONS) && defined(MBEDTLS_FS_IO)
 /* Default implementations for the platform independent seed functions use