Make private functions static and add missing prototypes.

Fixes:
mongoose/mongoose.c:180:8: warning: no previous prototype for ‘mg_dns_parse_name’ [-Wmissing-prototypes]
  180 | size_t mg_dns_parse_name(const uint8_t *s, size_t n, size_t ofs, char *dst,
      |        ^~~~~~~~~~~~~~~~~
mongoose/mongoose.c:306:6: warning: no previous prototype for ‘mg_dns_send’ [-Wmissing-prototypes]
  306 | void mg_dns_send(struct mg_connection *c, const struct mg_str *name,
      |      ^~~~~~~~~~~
mongoose/mongoose.c:925:6: warning: no previous prototype for ‘mg_http_parse_headers’ [-Wmissing-prototypes]
  925 | void mg_http_parse_headers(const char *s, const char *end,
      |      ^~~~~~~~~~~~~~~~~~~~~
mongoose/mongoose.c:1125:7: warning: no previous prototype for ‘mg_http_etag’ [-Wmissing-prototypes]
 1125 | char *mg_http_etag(char *buf, size_t len, size_t size, time_t mtime) {
      |       ^~~~~~~~~~~~
mongoose/mongoose.c:2578:6: warning: no previous prototype for ‘mg_sha1_transform’ [-Wmissing-prototypes]
 2578 | void mg_sha1_transform(uint32_t state[5], const unsigned char buffer[64]) {
      |      ^~~~~~~~~~~~~~~~~
mongoose/mongoose.c:2976:8: warning: no previous prototype for ‘mg_open_listener’ [-Wmissing-prototypes]
 2976 | SOCKET mg_open_listener(const char *url, struct mg_addr *addr) {
      |        ^~~~~~~~~~~~~~~~

Signed-off-by: James Hilliard <james.hilliard1@gmail.com>

test/mongoose_custom.c

@@ -23,6 +23,7 @@ struct mg_connection *mg_connect(struct mg_mgr *mgr, const char *url,
   return NULL;
 }
 
+void mg_connect_resolved(struct mg_connection *c);
 void mg_connect_resolved(struct mg_connection *c) {
   (void) c;
 }
@@ -52,5 +53,6 @@ struct mg_connection *mg_mkpipe(struct mg_mgr *mgr, mg_event_handler_t fn,
   return NULL;
 }
 
+void _fini(void);
 void _fini(void) {
 }