Update imgsmlr.h

[digoal]

fix error:

[digoal]

In file included from imgsmlr_idx.c:18:
imgsmlr_idx.c: In function ‘signature_compress’:
imgsmlr_idx.c:67:23: error: ‘FALSE’ undeclared (first use in this function)
   67 |        entry->offset, FALSE);
      |                       ^~~~~
/usr/include/postgresql/15/server/access/gist.h:246:37: note: in definition of macro ‘gistentryinit’
  246 |    (e).offset = (o); (e).leafkey = (l); } while (0)
      |                                     ^
imgsmlr_idx.c:67:23: note: each undeclared identifier is reported only once for each function it appears in
   67 |        entry->offset, FALSE);
      |                       ^~~~~
/usr/include/postgresql/15/server/access/gist.h:246:37: note: in definition of macro ‘gistentryinit’
  246 |    (e).offset = (o); (e).leafkey = (l); } while (0)
      |                                     ^
imgsmlr_idx.c: In function ‘signature_decompress’:
imgsmlr_idx.c:89:23: error: ‘FALSE’ undeclared (first use in this function)
   89 |        entry->offset, FALSE);
      |                       ^~~~~
/usr/include/postgresql/15/server/access/gist.h:246:37: note: in definition of macro ‘gistentryinit’
  246 |    (e).offset = (o); (e).leafkey = (l); } while (0)
      |                                     ^
make: *** [<builtin>: imgsmlr_idx.o] Error 1