maint: setupext.py for freetype had a Catch case for missing ft2build.h

The error was that the path was constructed manually with '\'. This is now fixed
to use os.path.join

Signed-off-by: Nick Papior <[email protected]>

Author: zerothi

setupext.py

@@ -906,7 +906,7 @@ def check(self):
             try:
                 check_include_file(get_include_dirs(), 'ft2build.h', 'freetype')
             except CheckFailed:
-                check_include_file(get_include_dirs(), 'freetype2\\ft2build.h', 'freetype')
+                check_include_file(get_include_dirs(), os.path.join('freetype2', 'ft2build.h'), 'freetype')
             return 'Using unknown version found on system.'
 
         status, output = subprocess.getstatusoutput(