librsvg: unpin to allow upgrade to v2.51.0+

[Lovell Fuller]

Will depend on libvips/libvips#1985 and therefore libvips v8.10.6

Also requires the following patch:

-# Do not include debugging symbols
-sed -i'.bak' "/debug =/ s/= .*/= false/" Cargo.toml
 # LTO optimization does not work for staticlib+rlib compilation
-sed -i'.bak' "s/, \"rlib\"//" librsvg/Cargo.toml
+sed -i'.bak' "s/, \"rlib\"//" Cargo.toml
 # Skip executables
-sed -i'.bak' "/PROGRAMS = /d" Makefile.in
+sed -i'.bak' "/SCRIPTS = /d" Makefile.in