Clang complains about the size of empty structs.

Change-Id: I2951b1b384cf9b32677aea81c66c34ab07b0090f
Reviewed-on: https://code.wireshark.org/review/1835
Reviewed-by: Michael Mann <mmann78@netscape.net>
Reviewed-by: Michael Tüxen <tuexen@wireshark.org>

capture_session.h

@@ -58,7 +58,8 @@ extern void
 capture_session_init(capture_session *cap_session, void *cf);
 #else
 
-typedef struct {} capture_session;
+/* dummy is needed because clang throws the error: empty struct has size 0 in C, size 1 in C++ */
+typedef struct {int dummy;} capture_session;
 
 #endif /* HAVE_LIBPCAP */