Resolve reported clang warning:

inkey.cpp:592:20: warning: implicit conversion from 'long' to 'int' changes value from 2147483648 to -2147483648 [-Wconstant-conversion]
         if(!term_supports(MOUSE))
             ~~~~~~~~~~~~~ ^~~~~
 ./sbbsdefs.h:574:19: note: expanded from macro 'MOUSE'
 #define MOUSE           (1L<<31)                /* Mouse supported terminal                             */