QDoc does not find any header file when headers.fileextensions is not declared

The documentation for QDoc states that headers.fileextensions defaults to some values which, for example, would find header files with a ".h" extensions.

Building a minimal project with an header file, let it be test.h, declaring a simple class, a qdoc or cpp file declaring some simple documentation for that class, let it be test.qdoc, and with the following minimal configuration, all siblings in the same directory:

project = Test
moduleheader = test.h
sources = test.qdoc

headerdirs = .
includepaths = .

And calling QDoc on the configuration file, will generate a warning that the class that is being documented cannot be found in any header file.

Using the `-debug` flag, we can see that QDoc does not parse the header file at all.

Adding an `headers.fileextensions = "*.h"` to the configuration will remove the warning and have QDoc parse the header file.

Standing by the documentation, it shouldn't be required to declare any fileextensions as an `.h` file should be found by the default values for it.