UseSWIG: track generated Perl module files

This makes sure the generated file is cleaned properly and allows to
create dependencies on it.

Help/release/dev/UseSWIG-perl-extra-files.rst

@@ -0,0 +1,5 @@
+UseSWIG-perl-extra-files
+------------------------
+
+* The :module:`UseSWIG` module gained knowledge about generated Perl module
+  files (extension ``.pm``).

Modules/UseSWIG.cmake

@@ -377,6 +377,7 @@ set(SWIG_EXTRA_LIBRARIES "")
 set(SWIG_PYTHON_EXTRA_FILE_EXTENSIONS ".py")
 set(SWIG_JAVA_EXTRA_FILE_EXTENSIONS ".java" "JNI.java")
 set(SWIG_CSHARP_EXTRA_FILE_EXTENSIONS ".cs" "PINVOKE.cs")
+set(SWIG_PERL_EXTRA_FILE_EXTENSIONS ".pm")
 
 set(SWIG_MANAGE_SUPPORT_FILES_SCRIPT "${CMAKE_CURRENT_LIST_DIR}/UseSWIG/ManageSupportFiles.cmake")