www/firefox: restore cbindgen < 0.6.7 support (after r483606)

dist/include/nsStyleStruct.h:1896:3: error: unknown type name 'StyleFillRule'
dist/include/nsStyleStruct.h:2134:12: error: no type named 'StyleDisplay' in namespace 'mozilla'

PR: 229893
Reported by: yuripv
Obtained from: upstream