libaom: fix arm configuration

arm_cpudetect.c is not included automatically.

BUG=aomedia:2294

Change-Id: Ia3cd70d050faed3ed88f94bb2b0f1e9eaf746a83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1502453Reviewed-by: James Zern <jzern@google.com>
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Cr-Commit-Position: refs/heads/master@{#637883}

third_party/libaom/BUILD.gn

@@ -161,7 +161,12 @@ if (enable_av1_decoder) {
         cflags = [ "-mfpu=neon" ]
       }
       configs += [ ":libaom_config" ]
-      sources = aom_av1_common_intrin_neon
+
+      # https://bugs.chromium.org/p/aomedia/issues/detail?id=2294
+      sources = [
+        "//third_party/libaom/source/libaom/aom_ports/arm_cpudetect.c",
+      ]
+      sources += aom_av1_common_intrin_neon
       sources += aom_dsp_common_intrin_neon
     }
   }