package/libplacebo: fix license

The license is actually LGPL-2.1+.

Fixes: 006aab8d64d2179d77df9a696b3fd8636f541640 ("package/libplacebo: add libplacebo package")
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/libplacebo/libplacebo.mk

@@ -8,7 +8,7 @@ LIBPLACEBO_VERSION = v7.351.0
 LIBPLACEBO_SITE = https://code.videolan.org/videolan/libplacebo.git
 LIBPLACEBO_SITE_METHOD = git
 LIBPLACEBO_GIT_SUBMODULES = YES
-LIBPLACEBO_LICENSE = LGPL-2.1
+LIBPLACEBO_LICENSE = LGPL-2.1+
 LIBPLACEBO_LICENSE_FILES = LICENSE
 LIBPLACEBO_INSTALL_STAGING = YES