Fix debug postfix in pkgconfig file

CMakeLists.txt uses set(CMAKE_DEBUG_POSTFIX "d") to distinguish
between debug and release lib. Use this postfix also in the
generated tinyxml2.pc file in case of a CMake Debug build.
This results in the following contents of tinyxml2.pc
- Release:
  Libs: -L${libdir} -ltinyxml2
- Debug:
  Libs: -L${libdir} -ltinyxml2d

tinyxml2.pc.in

@@ -6,5 +6,5 @@ includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
 Name: TinyXML2
 Description: simple, small, C++ XML parser
 Version: @GENERIC_LIB_VERSION@
-Libs: -L${libdir} -ltinyxml2
+Libs: -L${libdir} -ltinyxml2@LIB_POSTFIX@
 Cflags: -I${includedir}