cmake: Fix libdir value in pkgconfig file (#2407)

Depending on the OS the lib dir can vary, on Fedora for instance it is
"${prefix}/lib64". Instead of hard-coding the directory name, let CMake fill
this variable for us.

cmake/whisper.pc.in

@@ -1,6 +1,6 @@
 prefix=@CMAKE_INSTALL_PREFIX@
 exec_prefix=${prefix}
-libdir=${exec_prefix}/lib
+libdir=@CMAKE_INSTALL_FULL_LIBDIR@
 includedir=${prefix}/include
 
 Name: whisper