parser: fix file path into URI#open for Windows (#4855)

**Which issue(s) this PR fixes**: 
Fixes #

**What this PR does / why we need it**: 
This patch will fix the problem in
#4848
`u.path` has the file path without a drive letter in Windows
environment.

If Fluentd is worked on `c` drive and config file exists in `d` drive,
it can't parse config file, I think.

**Docs Changes**:

**Release Note**:

Signed-off-by: Shizuo Fujita <[email protected]>

Author: Watson1978

lib/fluent/config/parser.rb

@@ -107,7 +107,7 @@ def process_include(attrs, elems, uri, allow_include = true)
             #   - uri is 'c:/path/to/file'
             #   - u.path is '/path/to/file' and u.scheme is 'c'
             # Therefore, the condition of the if statement above is not met and it is handled here.
-            File.open(u.path, &parser_proc)
+            File.open(uri, &parser_proc)
           end
         end