Use mbedtls-based path for includes

To help the build system find the correct include files, paths starting
with "mbedtls/" or "psa/" must be used. Otherwise, you can run into
build failures like the following when building Mbed Crypto as a
submodule.

    In file included from chachapoly.c:31:0:
    ../../include/mbedtls/chachapoly.h:43:10: fatal error: poly1305.h: No such file or directory
     #include "poly1305.h"
              ^~~~~~~~~~~~
    compilation terminated.

Includes for ALT implementations are not modified, as the alt headers
are provided by system integrators and not Mbed TLS or Mbed Crypto.
diff --git a/include/mbedtls/platform_time.h b/include/mbedtls/platform_time.h
index 2ed36f5..fe484fd 100644
--- a/include/mbedtls/platform_time.h
+++ b/include/mbedtls/platform_time.h
@@ -25,7 +25,7 @@
 #define MBEDTLS_PLATFORM_TIME_H
 
 #if !defined(MBEDTLS_CONFIG_FILE)
-#include "config.h"
+#include "mbedtls/config.h"
 #else
 #include MBEDTLS_CONFIG_FILE
 #endif