package/grpc: add missing licenses

Some of the third party code is BSD-licensed. In addition, the roots.pem
certificate store is MPL-licensed.

This was probably already the case in earlier versions as well, but it
was only noticed while updating to 1.42.0 because the LICENSE file was
adapted for it.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/grpc/grpc.mk

@@ -6,7 +6,7 @@
 
 GRPC_VERSION = 1.41.0
 GRPC_SITE = $(call github,grpc,grpc,v$(GRPC_VERSION))
-GRPC_LICENSE = Apache-2.0
+GRPC_LICENSE = Apache-2.0, BSD-3-Clause (third_party code), MPL-2.0 (etc/roots.pem)
 GRPC_LICENSE_FILES = LICENSE
 GRPC_CPE_ID_VENDOR = grpc