examples: mbedtls: Fix cppcheck #include failure

Mbed TLS has two ways of overriding configurations:
* setting `MBEDTLS_CONFIG_FILE` to a full config file provided by the
user application
* setting `MBEDTLS_USER_CONFIG_FILE` to an override config file that
gets included at the end of Mbed TLS's default config file

As the Mbed TLS example uses `MBEDTLS_CONFIG_FILE` which is already
fully provided by the application, there is no point including
`MBEDTLS_USER_CONFIG_FILE`. The latter variable results in a cppcheck
failure because `#include MBEDTLS_USER_CONFIG_FILE` does not follow the
standard format of quoting a file.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>