BUILD: silence a warning on Solaris about usage of isdigit()

On Solaris, isdigit() is a macro and it complains about the use of
a char instead of the int for the argument. Let's cast it to an int
to silence it.
diff --git a/src/dumpstats.c b/src/dumpstats.c
index db82005..997bdec 100644
--- a/src/dumpstats.c
+++ b/src/dumpstats.c
@@ -322,7 +322,7 @@
 			else if (strcmp(args[cur_arg], "even") == 0) {
 				set |= 0xAAAAAAAA;
 			}
-			else if (isdigit(*args[cur_arg])) {
+			else if (isdigit((int)*args[cur_arg])) {
 				char *dash = strchr(args[cur_arg], '-');
 
 				low = high = str2uic(args[cur_arg]);