sqlite no longer builds on NetBSD/amd64 due mu.enter and mu.leave of lib/sqlite_netbsd_amd64.go undefined

When trying to build latest HEAD (currently commit 482dd8ac375853c5362828d40a96e6480bd92449 AKA tag v1.40.0) on NetBSD/amd64 11.99.3 of October 14th, 2025 with Go 1.25.3 the build fails as follows:

$ go build -v
modernc.org/sqlite/lib
# modernc.org/sqlite/lib
lib/sqlite_netbsd_amd64.go:112447:5: mu.enter undefined (type mutex has no field or method enter)
lib/sqlite_netbsd_amd64.go:112448:11: mu.leave undefined (type mutex has no field or method leave)

This was noticed while trying to package pkgsrc packages depending on modern.c/sqlite v1.39.0.