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.

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 ) );