check_GNU_style.py: Skip .md square bracket linting

This testcase causes lots of false-positives for machine description files.

contrib/ChangeLog:

	* check_GNU_style_lib.py: Skip machine description file bracket linting.

Signed-off-by: Patrick O'Neill <patrick@rivosinc.com>

contrib/check_GNU_style_lib.py

@@ -182,6 +182,9 @@ class SquareBracketCheck:
         self.re = re.compile('\w\s+(\[)')
 
     def check(self, filename, lineno, line):
+        if filename.endswith('.md'):
+            return None
+
         m = self.re.search(line)
         if m != None:
             return CheckError(filename, lineno,