signal.c: Use MINSIGSTKSZ by casting to int

MINSIGSTKSZ is unsigned int in OpenBSD, which makes a warning of
comparison:

```
signal.c:535:14: warning: comparison of integers of different signs: 'int' and 'unsigned int' [-Wsign-compare]
    if (size < MINSIGSTKSZ)
        ~~~~ ^ ~~~~~~~~~~~
1 warning generated.
```
https://rubyci.org/logs/rubyci.s3.amazonaws.com/openbsd-current/ruby-master/log/20191004T200009Z.log.html.gz

signal.c

@@ -532,8 +532,11 @@ rb_sigaltstack_size(void)
     int size = 16*1024;
 
 #ifdef MINSIGSTKSZ
-    if (size < MINSIGSTKSZ)
-	size = MINSIGSTKSZ;
+    {
+        int minsigstksz = (int)MINSIGSTKSZ;
+        if (size < minsigstksz)
+            size = minsigstksz;
+    }
 #endif
 #if defined(HAVE_SYSCONF) && defined(_SC_PAGE_SIZE)
     {