Fix libxml++ contrib library compiler options

Linking cgimap_common_compiler_options with INTERFACE scope is
incorrect as that only applies to targets using the library but not
the library itself.
(Copy paste error from catch2 which is an interface library and can
therefore only link to other libraries with INTERFACE scope)

Author: Woazboat

contrib/CMakeLists.txt

@@ -5,4 +5,4 @@ add_subdirectory(catch2)
 add_subdirectory(libxml++)
 
 target_link_libraries(catch2 INTERFACE cgimap_common_compiler_options)
-target_link_libraries(libxml++ INTERFACE cgimap_common_compiler_options)
+target_link_libraries(libxml++ PUBLIC cgimap_common_compiler_options)