Fix compilation with old versions of librsvg

* src/image.c (svg_load_image): Use LIBRSVG_CHECK_VERSION only
if it's defined; it isn't in old versions of librsvg.
1 job for master in 53 minutes and 8 seconds (queued for 2 seconds)
Status Job ID Name Coverage
failed #1702


Name Stage Failure
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
Makefile:293: recipe for target 'check-expensive' failed
make[1]: *** [check-expensive] Error 2
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:957: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1