Bug 1351803 - Statically link to avrt.lib on Windows, when compiling cubeb. r=glandium

We used to get the symbols we need using `LoadLibrary`, but now (since
661c653c86),
we can statically link to avrt.lib, because we only support Windows OSes that
have this library. For now, this works because avrt.lib is linked in
webrtc-land somewhere with a #pragma, but:
- We're going to stop using this bit of webrtc in the near future (around
  Firefox 54)
- Not linking this explicitely where it's used breaks --disable-webrtc, that is
  a popular option

MozReview-Commit-ID: 7b16Kdl3VUu

--HG--
extra : rebase_source : b4159872427b4e9d912e1228e75563ca9a8e828d

media/libcubeb/src/moz.build

@@ -68,6 +68,9 @@ if CONFIG['OS_TARGET'] == 'WINNT':
     ]
     DEFINES['USE_WINMM'] = True
     DEFINES['USE_WASAPI'] = True
+    OS_LIBS += [
+      "avrt",
+    ]
     if CONFIG['_MSC_VER']:
         CXXFLAGS += ['-wd4005'] # C4005: '_USE_MATH_DEFINES' : macro redefinition