sysutils/lnav: Fix Makefile error

Fix broken build:

[00:00:32] Warning: (sysutils/lnav): make: "/usr/ports/sysutils/lnav/Makefile" line 14: Unassociated shell command "https://github.com/tstack/lnav/issues/1328"
[00:00:32] Warning: (sysutils/lnav): make: Fatal errors encountered -- cannot continue
[00:00:32] Warning: (sysutils/lnav): Error: Error looking up dependencies for sysutils/lnav
[00:00:40] Error: Fatal errors encountered gathering ports metadata

Fixes: 5058ebb5ef38