BLOBNBOX: Declare signed bit field

This fixes a warning from LGTM:

    Bit field area of type int should have explicitly unsigned integral,
    explicitly signed integral, or enumeration type.

Maybe area should be unsigned, but that would require lots of other
changes, so for now signedness is not changed.

Signed-off-by: Stefan Weil <sw@weilnetz.de>

src/ccstruct/blobbox.h

@@ -519,7 +519,7 @@ class BLOBNBOX:public ELIST_LINK
   C_BLOB *cblob_ptr;            // edgestep blob
   TBOX box;                     // bounding box
   TBOX red_box;                 // bounding box
-  int area:30;                  // enclosed area
+  signed int area:30;           // enclosed area
   unsigned joined : 1;          // joined to prev
   unsigned reduced : 1;         // reduced box set
   int repeated_set_;            // id of the set of repeated blobs