intprops.h needs multi-inclusion guard

[Bruce Korb]

In file included from ../lib/inttostr.h:24,
                 from shar.c:140:
../lib/intprops.h:44:1: warning: "TYPE_MINIMUM" redefined
shar.c:108:1: warning: this is the location of the previous definition
../lib/intprops.h:50:1: warning: "TYPE_MAXIMUM" redefined
shar.c:113:1: warning: this is the location of the previous definition