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 which are dependent on the configuration option MBEDTLS_PEM_WRITE_C. If the option isn't defined the build breaks. This change adds the compilation condition MBEDTLS_PEM_WRITE_C to these sample application.
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