Log Message:
devel/libaom: Require gcc 10 instead of 9

This is marked for 9 due to a gcc bug.  Require 10 to align with the
limited-versions guidance in mk/compiler/gcc.mk, avoiding building 9
on a NetBSD 9 system, which almost certainly has or will need 10
anyway.

Build tested on n9/i386 and n10/amd64.