[lldb] Explicitly import json in TestSymbolFileJSON.py

The test was relying on the json module getting imported transitively by
one of its imported modules. Make this less brittle by importing it
explicitly.

(cherry picked from commit 9c972a3)

Author: JDevlieghere

lldb/test/API/macosx/symbols/TestSymbolFileJSON.py

@@ -4,6 +4,8 @@
 from lldbsuite.test.lldbtest import *
 from lldbsuite.test import lldbutil
 
+import json
+
 
 class TargetSymbolsFileJSON(TestBase):