Bug 1833064 - Add icu headers needed for TimezoneDatabase.cpp to system headers. r=leftmostcat

Adding these two headers to the system headers list fixes a compile error when
building with --with-system-icu. Calendar functionality will be broken however
due to bug 1790071 (the upstream bug remains unfixed).

Differential Revision: https://phabricator.services.mozilla.com/D178379

--HG--
extra : moz-landing-system : lando

mail/app-system-headers.mozbuild

@@ -10,3 +10,9 @@
 #    'file1.h',
 #    'file2.h',
 # ]
+
+if CONFIG["MOZ_SYSTEM_ICU"]:
+    system_headers += [
+        "unicode/strenum.h",
+        "unicode/vtzone.h",
+    ]