Force-include assert.h for third_party/oboe

Oboe uses assert but doesn't seem to include the header explicitly:

../../third_party/externals/oboe/src/flowgraph/resampler/PolyphaseResamplerStereo.cpp:25:5: error: use of undeclared identifier 'assert'
    assert(builder.getChannelCount() == STEREO);

Change-Id: Ib008ffaeade0b84196141102b956bc6e5606a468
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/383698
Reviewed-by: Jorge Betancourt <jmbetancourt@google.com>
Commit-Queue: Florin Malita <fmalita@google.com>

third_party/oboe/BUILD.gn

@@ -5,7 +5,14 @@
 
 import("../third_party.gni")
 
+config("oboe_lib") {
+  # Oboe does not include this explicitly
+  cflags = [ "--includeassert.h" ]
+}
+
 third_party("oboe") {
+  configs += [ ":oboe_lib" ]
+
   public_include_dirs = [
     "../externals/oboe/include",
     "../externals/oboe/samples/parselib/src/main/cpp",