cairo: PDF support is needed when SVG support is enabled

As soon as PostScript, PNG or SVG support is enabled, PDF support is
required for Cairo to build properly. Otherwise, you get build
failures such as:

.libs/cairo-type3-glyph-surface.o: In function `_cairo_type3_glyph_surface_set_stream':
/home/thomas/local/buildroot-dl/cairo-1.8.10/src/cairo-type3-glyph-surface.c:337: undefined reference to `_cairo_pdf_operators_set_stream'
/home/thomas/local/buildroot-dl/cairo-1.8.10/src/cairo-type3-glyph-surface.c:337: undefined reference to `_cairo_pdf_operators_set_stream'

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>