Add Windows builders for libaom and brotli libraries

Both libraries are listed in lib.json and used as transitive dependencies
(libaom via libavif, brotli via freetype/curl) but had no Windows builder,
causing builds to fail with "library [X] is in the lib.json list but not
supported to compile".

libaom: Uses builddir instead of build to avoid collision with the
source tree's build/cmake/ directory. Matches the Unix builder's
AOM_TARGET_CPU=generic setting for portability.

brotli: Standard CMake build with shared libs and tools disabled.

Also adds static-libs-windows entry for libaom in lib.json.

config/lib.json

@@ -355,6 +355,9 @@
         "static-libs-unix": [
             "libaom.a"
         ],
+        "static-libs-windows": [
+            "aom.lib"
+        ],
         "cpp-library": true
     },
     "libargon2": {