Makefile.am: update library name to avoid conflict with librsvg
With this change, installing libsvgtiny-pixbuf alongside librsvg
should be possible. The mime types still "conflict," but the librsvg
library will come last alphabetically and should take therefore take
precendence (source: I tried it both ways). As for the thumbnailers:
it doesn't matter which gets run. They both literally do the same
thing.
The test suite also continues to exercise the correct library, even
when librsvg is installed. It seems that GDK_PIXBUF_MODULE_FILE takes
precedence over the system loader cache.