[cmake] static build: populate private

in pkg-config files only add Requires.private and Libs.private if we
build a static library.

winpr/winpr.pc.in

@@ -11,5 +11,5 @@ Version: @WINPR_VERSION@
 Requires:
 Requires.private: @WINPR_PC_REQUIRES_PRIVATE@
 Libs: -L${libdir} ${libs}
-Libs.private: -ldl -lrt -lm -lpthread
+Libs.private: @WINPR_PC_LIBRARY_PRIVATE@
 Cflags: -I${includedir}