Add const to strings in skeletons/

This removes the GCC warning:
"warning: initialization discards ‘const’ qualifier from pointer
target type [-Wdiscarded-qualifiers]"
from BIT_STRING.c and ber_tlv_tag.c.
diff --git a/skeletons/ber_tlv_tag.c b/skeletons/ber_tlv_tag.c
index 4270876..4a7d732 100644
--- a/skeletons/ber_tlv_tag.c
+++ b/skeletons/ber_tlv_tag.c
@@ -74,7 +74,7 @@
 
 ssize_t
 ber_tlv_tag_snprint(ber_tlv_tag_t tag, char *buf, size_t size) {
-	char *type = 0;
+	const char *type = 0;
 	int ret;
 
 	switch(tag & 0x3) {