package wavpack: add missing exec_prefix in wavpack.pc

I get a build error in the xine-lib package after the latest wavpack
update because exec_prefix is not defined in wavpack.pc.

src/wavpack-1-exec_prefix.patch

@@ -0,0 +1,14 @@
+This file is part of MXE.
+See index.html for further information.
+
+Make sure exec_prefix is defined in wavpack.pc.
+
+diff -ur a/wavpack.pc.in b/wavpack.pc.in
+--- a/wavpack.pc.in	2013-09-29 23:30:14.000000000 +0200
++++ b/wavpack.pc.in	2013-11-17 18:31:35.351567596 +0100
+@@ -1,4 +1,5 @@
+ prefix=@prefix@
++exec_prefix=${prefix}
+ libdir=@libdir@
+ includedir=${prefix}/include
+