rsvg2: suppress a warning

    /usr/include/librsvg-2.0/librsvg/librsvg-enum-types.h:5:2: warning: #warning "Including <librsvg/librsvg-enum-types.h> directly is deprecated." [-Wcpp]

The deprecation was introduced at librsvg 2.36.2. From NEWS in librsvg:

    Version 2.36.2
    ...
    - deprecate including headers apart from rsvg.h directly
    ...