misc: Fix warning when compiling telnet_interface.c
In file included from telnet_interface.c:30:0:
osmocom/core/socket.h:25:4: warning: 'struct osmo_fd' declared inside parameter list [enabled by default]
telnet_interface.c: In function 'telnet_init_dynif':
telnet_interface.c:84:4: warning: passing argument 1 of 'osmo_sock_init_ofd' from incompatible pointer type [enabled by default]
osmocom/core/socket.h:24:5: note: expected 'struct osmo_fd *' but argument is of type 'struct osmo_fd *'
diff --git a/include/osmocom/core/socket.h b/include/osmocom/core/socket.h
index c5288df..8821463 100644
--- a/include/osmocom/core/socket.h
+++ b/include/osmocom/core/socket.h
@@ -12,6 +12,7 @@
#include <stdint.h>
struct sockaddr;
+struct osmo_fd;
/* flags for osmo_sock_init. */
#define OSMO_SOCK_F_CONNECT (1 << 0)