Add extra compilation conditions to X.509 samples

The sample applications programs/pkey/cert_req.c and
programs/pkey/cert_write.c use the library functions mbedtls_pk_write_csr_pem()
and mbedtls_pk_write_crt_pem() respectively and programs/pkey/gen_key.c uses
the library function mbedtls_pk_write_key_pem().

These are dependent on the configuration option POLARSSL_PEM_WRITE_C. If the
option isn't defined the build breaks.

This change adds the compilation condition POLARSSL_PEM_WRITE_C to these
sample applications.
3 files changed