delete.c: Fix signedness issue in confirmation selection

'char' may be unsigned on some architectures, which will cause the
"overflow check" on decrement to fail.

This would at most result in a confusing UI issue where no confirmation
option appears to be selected.

src/delete.c

@@ -37,7 +37,8 @@
 
 
 static struct dir *root, *nextsel, *curdir;
-static char noconfirm = 0, ignoreerr = 0, state, seloption;
+static char noconfirm = 0, ignoreerr = 0, state;
+static signed char seloption;
 static int lasterrno;