Bug 1899356 - Link to winmm from widget/windows. r=glandium

Presumably in non-mingw builds it gets linked from somewhere else.

nsSound.cpp uses it so we need it. Before the regressing bug it used to
be linked from:

  https://hg.mozilla.org/integration/autoland/rev/6b167bbce57c#l9.30

Differential Revision: https://phabricator.services.mozilla.com/D211899

widget/windows/moz.build

@@ -199,6 +199,7 @@ OS_LIBS += [
     "ktmw32",
     "rpcrt4",
     "urlmon",
+    "winmm",
 ]
 
 # mingw is missing Windows toast notification definitions.