thrift: default thrift_thrift_includes to ['.']

In a lint context, it's useful to assume that included files sit next to
the current file by default. Users can still further customize this
configuration variable to add more include paths.

ale_linters/thrift/thrift.vim

@@ -2,7 +2,7 @@
 
 call ale#Set('thrift_thrift_executable', 'thrift')
 call ale#Set('thrift_thrift_generators', ['cpp'])
-call ale#Set('thrift_thrift_includes', [])
+call ale#Set('thrift_thrift_includes', ['.'])
 call ale#Set('thrift_thrift_options', '-strict')
 
 function! ale_linters#thrift#thrift#GetCommand(buffer) abort