src/usermod.c: Fix const correctness

Now that we use liba2i's const-generic macros, we can (and must) use a
'const char **' endp where the input string is 'const char *'.

Signed-off-by: Alejandro Colomar <[email protected]>

src/usermod.c

@@ -304,7 +304,7 @@ struct ulong_range
 
 static struct ulong_range getulong_range(const char *str)
 {
-	char                *pos;
+	const char          *pos;
 	unsigned long       first, last;
 	struct ulong_range  result = { .first = ULONG_MAX, .last = 0 };