Add "inc" as sign that it's the root of the library (#4093)

* Add "inc" as sign that it's the root of the library

* Add "inc" and "Inc"

Co-authored-by: Ivan Kravets <me@ikravets.com>

platformio/package/manager/library.py

@@ -62,7 +62,7 @@ class LibraryPackageManager(BasePackageManager):  # pylint: disable=too-many-anc
 
     @staticmethod
     def find_library_root(path):
-        root_dir_signs = set(["include", "Include", "src", "Src"])
+        root_dir_signs = set(["include", "Include", "inc", "Inc", "src", "Src"])
         root_file_signs = set(
             [
                 "conanfile.py",  # Conan-based library