spec: use Fedora "LGPLv2+" license identifier
rpmlint complains that "LGPL" is not a valid License tag.  The License
tag for "GNU Lesser General Public License v2 (or 2.1) or later" is
"LGPLv2+" according to:
@@ -2,7 +2,7 @@ Name: liburing
Version: 0.1
Release: 1
Summary: Linux-native io_uring I/O access library
-License: LGPL
+License: LGPLv2+
Group: System Environment/Libraries
Source: %{name}-%{version}.tar.gz
BuildRoot: %{_tmppath}/%{name}-root
