audio/csound: Unbreak build after r500286

> Installing existing package /packages/All/stk-4.6.1.txz

[112i386-default-job-07] Installing stk-4.6.1...
[112i386-default-job-07] Extracting stk-4.6.1: .......... done

> csound-6.12.2.83_4 depends on shared library: libstk-4.6.0.so - not found

Use version independent libstk.so symlink in LIB_DEPENDS, so that
it does not immediately break on audio/stk updates in the future.
Also bump revision for shared library name change
(libstk-4.6.0.so->libstk-4.6.1.so).