[Var] Update gitignore.

git-svn-id: https://source.openmpt.org/svn/openmpt/trunk/OpenMPT@25480 56274372-70c3-4bfc-bfc3-4c3a0b034d27

Author: manxorist

.gitignore

@@ -73,6 +73,8 @@
 /build/vs2022win81/*.user
 /build/vs2022win81/.vs
 /build/vs2022win81/ext/*.user
+/build/vs2026previewwin11/*.user
+/build/vs2026previewwin11/.vs
 /build/vs2026win11/*.user
 /build/vs2026win11/.vs
 /build/vs2026win11clang/*.user