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 <kamil@netbsd.org> (maintainer:NETBSD)
 qemu-devel@nongnu.org (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 <f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>