Fix build without MBEDTLS_FS_IO

Fix missing definition of mbedtls_zeroize when MBEDTLS_FS_IO is
disabled in the configuration.

Introduced by e2985323947112a75c7c512ad062aa94da24cdbe
    Merge remote-tracking branch 'upstream-public/pr/1113' into mbedtls-1.3
diff --git a/library/pkparse.c b/library/pkparse.c
index 6fb0dd5..5786db7 100644
--- a/library/pkparse.c
+++ b/library/pkparse.c
@@ -61,12 +61,15 @@
 #define polarssl_free       free
 #endif
 
-#if defined(POLARSSL_FS_IO)
+#if defined(POLARSSL_FS_IO) || \
+    defined(POLARSSL_PKCS12_C) || defined(POLARSSL_PKCS5_C)
 /* Implementation that should never be optimized out by the compiler */
 static void polarssl_zeroize( void *v, size_t n ) {
     volatile unsigned char *p = v; while( n-- ) *p++ = 0;
 }
+#endif
 
+#if defined(POLARSSL_FS_IO)
 /*
  * Load all data from a file into a given buffer.
  */