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.

Without this patch, you get the following compilation error:

c:\miniconda\conda-bld\libsoup_1595835290538\work\libsoup\soup-brotli-decompressor.h(35): error C2375: 'soup_brotli_decompressor_get_type': redefinition; different linkage
c:\miniconda\conda-bld\libsoup_1595835290538\work\libsoup\soup-brotli-decompressor.h(29): note: see declaration of 'soup_brotli_decompressor_get_type'