assimp.pc.in: Add static libs to Libs.private

assimp.pc.in

@@ -7,4 +7,5 @@ Name: @CMAKE_PROJECT_NAME@
 Description: Import various well-known 3D model formats in an uniform manner.
 Version: @PROJECT_VERSION@
 Libs: -L${libdir} -lassimp@ASSIMP_LIBRARY_SUFFIX@
+Libs.private: @LIBSTDC++_LIBRARIES@ @ZLIB_LIBRARIES_LINKED@
 Cflags: -I${includedir}