Fix TypeError in default_opengles, default_sdl2

Commit 452d58a0b5 switched from passing a list of strings as the
right-hand side of `in` to passing a parenthesized string.  The new form
causes a TypeError if the left hand side is None:

```
>>> None in ('1', '2')
False
>>> None in ('1')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: 'in <string>' requires string as left operand, not NoneType
```

Add a trailing comma to turn the right hand side back into a list of
strings, as it was before the Raspberry Pi commit.

Fixes: 452d58a0b5 ("Update Raspberry Pi Mesa build for Buster & Pi 4B")

SConstruct

@@ -3534,11 +3534,11 @@ class DXXCommon(LazyObjectConstructor):
 			return self.debug
 		# automatic setup for raspberrypi
 		def default_opengles(self):
-			if self.raspberrypi in ('yes'):
+			if self.raspberrypi in ('yes',):
 				return True
 			return False
 		def default_sdl2(self):
-			if self.raspberrypi in ('mesa'):
+			if self.raspberrypi in ('mesa',):
 				return True
 			return False
 		@classmethod