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
MBEDTLS_PEM_PARSE_C.
diff --git a/library/pem.c b/library/pem.c
index d1c6604..8dd86a4 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -44,12 +44,12 @@
 #define mbedtls_free       free
 #endif
 
+#if defined(MBEDTLS_PEM_PARSE_C)
 /* Implementation that should never be optimized out by the compiler */
 static void mbedtls_zeroize( void *v, size_t n ) {
     volatile unsigned char *p = v; while( n-- ) *p++ = 0;
 }
 
-#if defined(MBEDTLS_PEM_PARSE_C)
 void mbedtls_pem_init( mbedtls_pem_context *ctx )
 {
     memset( ctx, 0, sizeof( mbedtls_pem_context ) );
diff --git a/library/x509_crt.c b/library/x509_crt.c
index 80af725..056dc16 100644
--- a/library/x509_crt.c
+++ b/library/x509_crt.c
@@ -969,8 +969,8 @@
  */
 int mbedtls_x509_crt_parse( mbedtls_x509_crt *chain, const unsigned char *buf, size_t buflen )
 {
-    int success = 0, first_error = 0, total_failed = 0;
 #if defined(MBEDTLS_PEM_PARSE_C)
+    int success = 0, first_error = 0, total_failed = 0;
     int buf_format = MBEDTLS_X509_FORMAT_DER;
 #endif
 
diff --git a/library/x509_csr.c b/library/x509_csr.c
index 603d06b..f92b66c 100644
--- a/library/x509_csr.c
+++ b/library/x509_csr.c
@@ -265,8 +265,8 @@
  */
 int mbedtls_x509_csr_parse( mbedtls_x509_csr *csr, const unsigned char *buf, size_t buflen )
 {
-    int ret;
 #if defined(MBEDTLS_PEM_PARSE_C)
+    int ret;
     size_t use_len;
     mbedtls_pem_context pem;
 #endif