Fix vi build on Linux/macOS

This absolute include causes a build failure on Linux for me:
.../cheri/freebsd/contrib/nvi/cl/../common/common.h:10:10: fatal error: '/usr/include/db.h' file not found

This change patches the file to use #include <db.h> instead until a
solution has been found upstream. See also https://github.com/lichray/nvi2/issues/69

Reviewed By: bapt
Differential Revision: https://reviews.freebsd.org/D26480