spice-server.pc.in: move Requires to Requires.private

When using pkg-config, Requires and Requires.private field list
packages required by this package, but packages listed under
Requires.private are not taken into account when a flag list is
computed for dynamically linked executable. In the situation
where each .pc file corresponds to a library, Requires.private
shall be used exclusively to specify the dependencies between
the libraries.

Author: liangguo

spice-server.pc.in

@@ -7,7 +7,7 @@ Name: spice
 Description: SPICE server library
 Version: @VERSION@
 
-Requires: @SPICE_REQUIRES@
+Requires.private: @SPICE_REQUIRES@
 Libs: -L${libdir} -lspice-server
 Libs.private: @SPICE_NONPKGCONFIG_LIBS@
 Cflags: -I${includedir}/spice-server