Greasemonkey: fix default include value

Greasemonkey scripts are supposed to default to running on all pages.
@jgkamat and @nemanjan00 repurted some script not running on all pages
unless they either removed (or broke) the metadata block or added an
include directive. Indeed I had a logic error when it only defaulted to
being included on all pages when no metadata block at all was included.
Whoops.

qutebrowser/browser/greasemonkey.py

@@ -92,7 +92,7 @@ class GreasemonkeyScript:
             props = ""
         script = cls(re.findall(cls.PROPS_REGEX, props), source)
         script.script_meta = props
-        if not props:
+        if not script.includes:
             script.includes = ['*']
         return script