soup-brotli-decompressor.h: remove superfluous get_type declaration

This fixes compilation with Visual Studio, as the explicit declaration
of soup_brotli_decompressor_get_type and the one generated by G_DECLARE_FINAL_TYPE
are apparently not identical for the cl.exe compiler.
1 job for master in 1 minute and 5 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #839315
ipv6
fedora-meson-x86_64

00:01:05