build: add License variable to pkg-config file

The pkg-config file has License variable that allows you to set the license
for the software. This sets 'LGPL-2.1-only' as defined in SPDX to License.

Ref: https://github.com/pkgconf/pkgconf/blob/master/man/pc.5#L116
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Acked-by: Tom Hromatka <tom.hromatka@oracle.com>
[PM: tweaked subj, line wrapping in description]
Signed-off-by: Paul Moore <paul@paul-moore.com>

Author: iwamatsu

libseccomp.pc.in

@@ -30,3 +30,4 @@ URL: https://github.com/seccomp/libseccomp
 Version: @PACKAGE_VERSION@
 Cflags: -I${includedir}
 Libs: -L${libdir} -lseccomp
+License: LGPL-2.1-only