main/postgresql-bdr-extension: fix wrong license

It seems that BDR has never been licensed under BSD, see [1] and [2].

[1]: https://github.com/2ndQuadrant/bdr-postgres/commit/9848d3655d44aa2e58d28fe9f93a94b2934eedc8
[2]: https://github.com/2ndQuadrant/bdr-postgres/blob/bdr-pg/REL9_4_STABLE/COPYRIGHT

main/postgresql-bdr-extension/APKBUILD

 # Maintainer: Leonardo Arena <rnalrd@alpinelinux.org>
 pkgname=postgresql-bdr-extension
 pkgver=1.0.2
-pkgrel=1
+pkgrel=2
 pkgdesc="PostgreSQL bidirectional replication (BDR) extension"
 url="https://wiki.postgresql.org/wiki/BDR_Administration"
 arch="all"
-license="GPL"
+license="PostgreSQL"
 makedepends="$depends_dev postgresql-bdr-dev"
 subpackages="$pkgname-doc"
 #FIXME: mkdir: can't create directory 'tmp_check/data': File exists