Export rsvg_handle_read_stream_sync

This is a litter weird, the CI seemed to have failed to build
due to the function not being exported, but locally it built
fine regardless of the environment and the rustc version. It
probably was a caching issue.

In any case, 9cb6c084 portted
rsvg_handle_read_stream_sync to Rust but did not export it
leading to the following linker error in the CI:

warning: function is marked #[no_mangle], but not exported
   --> rsvg_internals/src/handle.rs:646:1
    |
646 | / pub unsafe extern "C" fn rsvg_handle_rust_read_stream_sync(
647 | |     handle: *mut RsvgHandle,
648 | |     stream: *mut gio_sys::GInputStream,
649 | |     cancellable: *mut gio_sys::GCancellable,
...   |
671 | |     }
672 | | }
    | |_^
    |
    = note: #[warn(private_no_mangle_fns)] on by default
    = help: try exporting the item with a `pub use` statement

    Finished dev [unoptimized + debuginfo] target(s) in 1m 04s
  CCLD     librsvg-2.la

*** Warning: Linking the shared library librsvg-2.la against the
*** static library /builds/GNOME/librsvg/_build/target/debug/librsvg_internals.a is not portable!
  CCLD     rsvg-convert
/usr/bin/ld: ./.libs/librsvg-2.so: undefined reference to `rsvg_handle_rust_read_stream_sync'
collect2: error: ld returned 1 exit status
7 jobs from alatiera/pub-export in 27 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #170903
fedora test

00:06:58

 
  Lint
manual #170905
allowed to fail manual
clippy
passed #170904
rustfmt

00:01:19

passed #170906
tarball

00:12:29

 
  Cross Distro
passed #170909
debian_i386:testing

00:07:58

passed #170908
debian:testing

00:07:12

passed #170907
fedora:rawhide

00:07:25