utils: add osmo_strnchr()

When finding a char in a string, I want to be able to limit the search area by
size, not only by nul terminator.

Change-Id: I48f8ace9f51f8a06796648883afcabe3b4e8b537
diff --git a/include/osmocom/core/utils.h b/include/osmocom/core/utils.h
index 86d45bc..1abbe98 100644
--- a/include/osmocom/core/utils.h
+++ b/include/osmocom/core/utils.h
@@ -139,6 +139,7 @@
 uint8_t *osmo_encode_big_endian(uint64_t value, size_t data_len);
 
 size_t osmo_strlcpy(char *dst, const char *src, size_t siz);
+const char *osmo_strnchr(const char *str, size_t str_size, char c);
 
 bool osmo_is_hexstr(const char *str, int min_digits, int max_digits,
 		    bool require_even);