libsals2 dependency missing in libmemcached.pc

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As in installed headers we have

# include <sasl/sasl.h>

The libmemcached.pc should have this dependency

Requires: libsasl2

This will also fix Fedora bug #2002541 as such dependency are properly manage and detect during package build.

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Notice: I not a cmake expert, and don't know how to add such line with the proper condition... ;)

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Thanks!