package/librsvg: disable gdk-pixbuf-loader

package/librsvg: disable gdk-pixbuf-loader

gdk-pixbuf-loader support is enabled by default but it silently fail to
generate gdk-pixbuf.loaders file when host != target.

For exemple on ARM target:
output/host/usr/bin/gdk-pixbuf-query-loaders ./libpixbufloader-svg.la
g_module_open() failed for output/build/librsvg-2.40.16/gdk-pixbuf-loader/./libpixbufloader-svg.la: output/build/librsvg-2.40.16/gdk-pixbuf-loader/./.libs/libpixbufloader-svg.so: wrong ELF class: ELFCLASS32

But it doesn't break the build.

When host = target using the Sourcery CodeBench AMD64 2016.11 toolchain
optimized for x86_68 AMD Puma/Jaguar or AMD Steamroller, it break the
build due to "Illegal instruction".

output/host/usr/bin/gdk-pixbuf-query-loaders libpixbufloader-svg.la
Illegal instruction (core dumped)

Since this option is broken for cross-compilation, disable it.

Fixes:
http://autobuild.buildroot.net/results/393/393145bc9bcb93d6df55ec8c63725c3d9a299957

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>