MAINTAINERS: Improve the NetBSD regex pattern

MAINTAINERS: Improve the NetBSD regex pattern

Currently get_maintainers.pl claims that the configure script is
maintained by Kamil:

 $ scripts/get_maintainer.pl -f configure
 Kamil Rytarowski <[email protected]> (maintainer:NETBSD)
 [email protected] (open list:All patches CC here)

This happens because the regex pattern for the NETBSD entry triggers
on everything that contains the keyword "NetBSD". Ease the situation
a little bit by restricting this to "Subject:" lines only, like
we do it in the "trivial patches" section already.

Reported-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Thomas Huth <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Signed-off-by: Michael Tokarev <[email protected]>