treewide: Change .*GPL.*+ licenses to SPDX compatible identifier

The CONTRIBUTING.md requests an (or multiple) SPDX identifier for GPL
licenses. But a lot of packages did use a different, non-SPDX style with a
"+" at the end instead of "-or-later".

Signed-off-by: Sven Eckelmann <sven@narfation.org>
(cherry-picked from bbb1ea7345)

libs/fftw3/Makefile

@@ -9,8 +9,8 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=fftw3
 PKG_VERSION:=3.3.8
-PKG_RELEASE:=1
-PKG_LICENSE:=GPL-2.0+
+PKG_RELEASE:=2
+PKG_LICENSE:=GPL-2.0-or-later
 
 PKG_SOURCE:=fftw-$(PKG_VERSION).tar.gz
 PKG_SOURCE_URL:=http://www.fftw.org