Fix openh264 build config error.

Before this CL, the following GN args were causing an error [1] when
building WebRTC.

is_debug = false
target_cpu = "arm64"
target_os = "android"
rtc_use_h264=true

[1] -

ERROR at //third_party/openh264/BUILD.gn:38:5: Undefined identifier.
    defines += [ "ANDROID_NDK" ]
    ^------
See //modules/video_coding/BUILD.gn:325:7: which caused the file to be included.
      "//third_party/openh264:encoder",

Bug: None
Change-Id: Ia27d995500364306edc555132dba3dbfdf632daa
Reviewed-on: https://chromium-review.googlesource.com/c/1286409Reviewed-by: Henrik Boström <hbos@chromium.org>
Commit-Queue: Mirko Bonadei <mbonadei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600872}

third_party/openh264/BUILD.gn

@@ -10,11 +10,12 @@ import("//third_party/yasm/yasm_assemble.gni")
 # Config shared by all openh264 targets.
 config("config") {
   cflags = []
+  defines = []
 
   if (is_chromeos && target_cpu == "arm") {
     # HAVE_NEON and __chromeos__ are needed for enabling NEON on ChromeOS
     # devices.
-    defines = [
+    defines += [
       "HAVE_NEON",
       "__chromeos__",
     ]