submodule: add versions/26.01 (ip7z/7zip tag 26.01)

Registers the upstream ip7z/7zip repository as a git submodule at
versions/26.01, checked out at tag 26.01 (commit 8c63d71).

This provides the 7-Zip 26.01 vendor source tree for both the Windows
(MSBuild/vcxproj) and Linux (MinGW cross-compilation) builds.

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "versions/7-zip-zstd"]
 	path = versions/7-zip-zstd
 	url = https://github.com/mcmilk/7-Zip-zstd.git
+[submodule "versions/26.01"]
+	path = versions/26.01
+	url = https://github.com/ip7z/7zip.git