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.
diff --git a/programs/x509/cert_req.c b/programs/x509/cert_req.c
index 5cafb80..30df216 100644
--- a/programs/x509/cert_req.c
+++ b/programs/x509/cert_req.c
@@ -34,7 +34,8 @@
 
 #if !defined(MBEDTLS_X509_CSR_WRITE_C) || !defined(MBEDTLS_FS_IO) ||  \
     !defined(MBEDTLS_PK_PARSE_C) || !defined(MBEDTLS_SHA256_C) || \
-    !defined(MBEDTLS_ENTROPY_C) || !defined(MBEDTLS_CTR_DRBG_C)
+    !defined(MBEDTLS_ENTROPY_C) || !defined(MBEDTLS_CTR_DRBG_C) || \
+    !defined(MBEDTLS_PEM_WRITE_C)
 int main( void )
 {
     mbedtls_printf( "MBEDTLS_X509_CSR_WRITE_C and/or MBEDTLS_FS_IO and/or "
@@ -341,4 +342,4 @@
     return( ret );
 }
 #endif /* MBEDTLS_X509_CSR_WRITE_C && MBEDTLS_PK_PARSE_C && MBEDTLS_FS_IO &&
-          MBEDTLS_ENTROPY_C && MBEDTLS_CTR_DRBG_C */
+          MBEDTLS_ENTROPY_C && MBEDTLS_CTR_DRBG_C && MBEDTLS_PEM_WRITE_C */
diff --git a/programs/x509/cert_write.c b/programs/x509/cert_write.c
index 7907d82..66e5f1d 100644
--- a/programs/x509/cert_write.c
+++ b/programs/x509/cert_write.c
@@ -32,10 +32,11 @@
 #define mbedtls_printf     printf
 #endif
 
-#if !defined(MBEDTLS_X509_CRT_WRITE_C) ||                                  \
-    !defined(MBEDTLS_X509_CRT_PARSE_C) || !defined(MBEDTLS_FS_IO) ||      \
-    !defined(MBEDTLS_ENTROPY_C) || !defined(MBEDTLS_CTR_DRBG_C) ||        \
-    !defined(MBEDTLS_ERROR_C) || !defined(MBEDTLS_SHA256_C)
+#if !defined(MBEDTLS_X509_CRT_WRITE_C) || \
+    !defined(MBEDTLS_X509_CRT_PARSE_C) || !defined(MBEDTLS_FS_IO) || \
+    !defined(MBEDTLS_ENTROPY_C) || !defined(MBEDTLS_CTR_DRBG_C) || \
+    !defined(MBEDTLS_ERROR_C) || !defined(MBEDTLS_SHA256_C) || \
+    !defined(MBEDTLS_PEM_WRITE_C)
 int main( void )
 {
     mbedtls_printf( "MBEDTLS_X509_CRT_WRITE_C and/or MBEDTLS_X509_CRT_PARSE_C and/or "
@@ -664,4 +665,4 @@
 }
 #endif /* MBEDTLS_X509_CRT_WRITE_C && MBEDTLS_X509_CRT_PARSE_C &&
           MBEDTLS_FS_IO && MBEDTLS_ENTROPY_C && MBEDTLS_CTR_DRBG_C &&
-          MBEDTLS_ERROR_C */
+          MBEDTLS_ERROR_C && MBEDTLS_PEM_WRITE_C */