harfbuzz: add missing cairo dependency

Today harfbuzz refused to upgrade, failing with the following configure error:

    checking for CAIRO... configure: error: Package requirements (cairo >= 1.8.0) were not met:
    
    No package 'cairo' found
    
    Consider adjusting the PKG_CONFIG_PATH environment variable if you
    installed software in a non-standard prefix.
    
    Alternatively, you may set the environment variables CAIRO_CFLAGS
    and CAIRO_LIBS to avoid the need to call pkg-config.
    See the pkg-config man page for more details.

I added a cairo build dep in the formula which allowed it to upgrade. Looking at the rest of the formula, I'm not sure what `--disable-dependency-tracking` is supposed to do, but it sure sounds like it's supposed to avoid exactly this problem...