package/libopenaptx: fix license

License is GPL-3.0+, not GPL-3.0 since the addition of the package in
commit 1e4050b2d0372e1b47b57cc4210e3da5cb8ec3de

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/libopenaptx/libopenaptx.mk

@@ -6,7 +6,7 @@
 
 LIBOPENAPTX_VERSION = 0.2.1
 LIBOPENAPTX_SITE = $(call github,pali,libopenaptx,$(LIBOPENAPTX_VERSION))
-LIBOPENAPTX_LICENSE = GPL-3.0
+LIBOPENAPTX_LICENSE = GPL-3.0+
 LIBOPENAPTX_LICENSE_FILES = COPYING
 LIBOPENAPTX_INSTALL_STAGING = YES