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.