www/firefox: unbreak after r519576

[style 0.0.1] ../.build/dist/include/mozilla/ServoStyleConsts.h:10840:20: error: class member cannot be redeclared
[style 0.0.1] ../.build/dist/include/mozilla/ServoStyleConsts.h:10827:20: note: previous definition is here
[style 0.0.1] ../.build/dist/include/mozilla/ServoStyleConsts.h:10843:21: error: class member cannot be redeclared
[style 0.0.1] ../.build/dist/include/mozilla/ServoStyleConsts.h:10830:21: note: previous definition is here
[...]
In file included from Unified_cpp_xpcom_base2.cpp:11:
In file included from xpcom/base/nsMemoryInfoDumper.cpp:14:
In file included from ../.build/dist/include/mozilla/dom/ContentParent.h:10:
In file included from ../.build/ipc/ipdl/_ipdlheaders/mozilla/dom/PContentParent.h:9:
In file included from ../.build/ipc/ipdl/_ipdlheaders/mozilla/dom/PContent.h:11:
In file included from ../.build/dist/include/ipc/IPCMessageUtils.h:36:
In file included from ../.build/dist/include/nsIWidget.h:21:
In file included from ../.build/dist/include/nsStyleConsts.h:17:
../.build/dist/include/mozilla/ServoStyleConsts.h:10840:20: error: class member cannot be redeclared

StyleWritingMode operator^(const StyleWritingMode& other) const {
                 ^

../.build/dist/include/mozilla/ServoStyleConsts.h:10827:20: note: previous definition is here

StyleWritingMode operator^(const StyleWritingMode& other) const {
                 ^

../.build/dist/include/mozilla/ServoStyleConsts.h:10843:21: error: class member cannot be redeclared

StyleWritingMode& operator^=(const StyleWritingMode& other) {
                  ^

../.build/dist/include/mozilla/ServoStyleConsts.h:10830:21: note: previous definition is here

StyleWritingMode& operator^=(const StyleWritingMode& other) {
                  ^