etc/mtree/BSD.include.dist: Remove libmilter-dev

When building with WITHOUT_SENDMAIL this result in a FreeBSD-libmilter-dev
package with only this directory and a dependacy on FreeBSD-libmilter which
doesn't exists.

Differential Revision:	https://reviews.freebsd.org/D54193
Fixes:		436618a427 ("etc/mtree: Add package tags for /usr/include")
Reviewed by:	ivy
Sponsored by:	Beckhoff Automation GMbH & Co. KG

etc/mtree/BSD.include.dist

@@ -280,7 +280,7 @@
     ..
     libipt
     ..
-    libmilter           tags=package=libmilter-dev
+    libmilter
     ..
     libxo               tags=package=runtime-dev
     ..