Build failure with librsvg older than 2.52

[Lastique]

Compiling ffmpeg 6.1 on Debian Bullseye with librsvg 2.50.3 fails with the following error:

[01m[Ksrc/libavcodec/librsvgdec.c:[m[K In function '[01m[Klibrsvg_decode_frame[m[K':
[01m[Ksrc/libavcodec/librsvgdec.c:93:5:[m[K [01;31m[Kerror: [m[K'[01m[Kviewport[m[K' undeclared (first use in this function)
   93 |     [01;31m[Kviewport[m[K.width = dimensions.width;
      |     [01;31m[K^~~~~~~~[m[K
[01m[Ksrc/libavcodec/librsvgdec.c:93:5:[m[K [01;36m[Knote: [m[Keach undeclared identifier is reported only once for each function it appears in

It looks like there is a missing librsvg version check in libavcodec/librsvgdec.c:93.

[Lastique]

A fix for the compilation error.