[cairo-ft-font.c] Fix wrong conditionalization in c0ed8ce1.

FT_PIXEL_MODE_BGRA is not a macro defined by cpp, but an enuerator value. Use another cpp macro FT_LOAD_COLOR introduced at the same time.

Please refer the bug report by Matthias: https://lists.cairographics.org/archives/cairo/2019-May/028881.html

Also please refer the commit log when FT_PIXEL_MODE_BGRA and FT_LOAD_COLOR were introduced. https://git.savannah.gnu.org/cgit/freetype/freetype2.git/commit/?id=760d342d37ec9b26420956e3421075d410571b65