util/normalize: scan more than just .cpp files

There's no reason why SIGNAL()/SLOT() usage should be limited to .cpp
files, and, indeed, running normalize on qt5.git with this change
applied produces additional hits in headers, Obj-C++ and qdoc files.

Also add some other file types that may contain SIGNAL/SLOT macros.

Change-Id: I65082bcf54e977e55f8cb1f95d7a84ede087c9d2
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>