Fix compiler warning

In file included from asn1helpers.c:14:0:
../include/asn1c/asn1helpers.h: In function ‘OCTET_STRING_noalloc’:
../include/asn1c/asn1helpers.h:26:9: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]
  s->buf = str;
         ^

Change-Id: Ie2fefe710090de779137c36b98239ef4a097b6dd

include/asn1c/asn1helpers.h

@@ -23,6 +23,6 @@ uint32_t asn1bitstr_to_u24(const BIT_STRING_t *in);
 
 static inline void OCTET_STRING_noalloc(OCTET_STRING_t *s, const uint8_t *str, int size)
 {
-	s->buf = str;
+	s->buf = (uint8_t *) str;
 	s->size = size;
 }