BR git explicitly, it is used in %prep

Previously, it was pulled in transitively,
that is no longer true.

  Executing(%prep): /bin/sh -e /var/tmp/rpm-tmp.k6RazT
  ...
  + /usr/bin/git init -q
  /var/tmp/rpm-tmp.k6RazT: line 42: /usr/bin/git: No such file or directory
  error: Bad exit status from /var/tmp/rpm-tmp.k6RazT (%prep)

abrt.spec

@@ -57,6 +57,7 @@ Release: 2%{?dist}
 License: GPLv2+
 URL: https://abrt.readthedocs.org/
 Source: https://github.com/abrt/%{name}/archive/%{version}/%{name}-%{version}.tar.gz
+BuildRequires: git-core
 BuildRequires: %{dbus_devel}
 BuildRequires: hostname
 BuildRequires: gtk3-devel