Moved some functions under defined to get rid of compiler warnings.
Functions moved under defines:
- mbedtls_x509_get_alg
- mbedtls_x509_get_alg_null
- mbedtls_x509_get_time
- mbedtls_x509_get_ext
- mbedtls_x509_sig_alg_gets
- mbedtls_x509_key_size_helper
Left one function (mbedtls_x509_write_names) as non static as it increased code size.
This allows to build the library + tests via `make` without
specifying `PTHREAD=1`, in which case the X.509 threading
test suite will be silently dropped.
This is analogous to the pre-existing handling of the example
application `ssl_pthread_server`, which is only build if `PTHREAD=1`
and silently dropped otherwise.