Fix unused variable/function compilation warnings

This PR fixes a number of unused variable/function compilation warnings
that arise when using a config.h that does not define the macro
POLARSSL_PEM_PARSE_C.
diff --git a/library/x509_csr.c b/library/x509_csr.c
index 558b078..9bdfe88 100644
--- a/library/x509_csr.c
+++ b/library/x509_csr.c
@@ -260,8 +260,8 @@
  */
 int x509_csr_parse( x509_csr *csr, const unsigned char *buf, size_t buflen )
 {
-    int ret;
 #if defined(POLARSSL_PEM_PARSE_C)
+    int ret;
     size_t use_len;
     pem_context pem;
 #endif