[MacPorts] #43663: wavpack: fix pkgconfig file.

[MacPorts]

#43663: wavpack: fix pkgconfig file.
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 Reporter:  ionic@…   |      Owner:  macports-tickets@…
     Type:  defect    |     Status:  new
 Priority:  Normal    |  Milestone:
Component:  ports     |    Version:
 Keywords:  haspatch  |       Port:  wavpack
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 Include ${exec_prefix}, which is used within ${libdir} due to variable
 expansion.

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Ticket URL: <https://trac.macports.org/ticket/43663>
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