Suppress zend_signals warnings

When running the tests in a debug build, I started seeing the following error:
```
Warning: zend_signal: handler was replaced for signal (X) after startup in
    Unknown on line 0
```

At the time of writing, I have no clue what is causing the issue, but I
started looking at the PDO Firebird driver and this issue was fixed
there as well. See:
php/php-src@a9253b0

This patch does the exact same thing.

Author: Jon4t4n

interbase.c

@@ -759,6 +759,11 @@ PHP_MINIT_FUNCTION(ibase)
 	php_ibase_events_minit(INIT_FUNC_ARGS_PASSTHRU);
 	php_ibase_service_minit(INIT_FUNC_ARGS_PASSTHRU);
 
+#ifdef ZEND_SIGNALS
+	// firebird replaces some signals at runtime, suppress warnings.
+	SIGG(check) = 0;
+#endif
+
 	return SUCCESS;
 }