gchar is not guaranteed to be signed; use gint8.

C does not guarantee that char is signed (that's why there's a keyword
"signed").  Use gint8, to indicate that it's not a character, it's an
8-bit signed integer.

Change-Id: Id632df23352840cbc86a4d88f4c7dade95ab7837
Reviewed-on: https://code.wireshark.org/review/4736
Reviewed-by: Guy Harris <guy@alum.mit.edu>

caputils/airpcap.h

@@ -284,7 +284,7 @@ typedef struct _AirpcapChannelInfo
 
         In case of 802.11a/b/g channels (802.11n legacy mode), this field should be set to 0.
     */
-    gchar ExtChannel;
+    gint8 ExtChannel;
     guint8 Reserved[3]; /* < Reserved. It should be set to {0,0,0}. */
 }
     AirpcapChannelInfo, *PAirpcapChannelInfo;