BUILD(cmake): Find and link Poco::XML

Without this, building against the latest poco snapshot results in
errors like:

/usr/bin/ld: /builddir/build/BUILD/mumble-1.4.230.src/src/mumble/PluginManifest.cpp:72: undefined reference to `typeinfo for Poco::XML::Element'

(cherry picked from commit fefdd79)

# Conflicts:
#	src/mumble/CMakeLists.txt

src/mumble/CMakeLists.txt

@@ -384,11 +384,16 @@ target_include_directories(mumble
 		"${PLUGINS_DIR}"
 )
 
-find_pkg(Poco COMPONENTS Zip)
+find_pkg(Poco
+	COMPONENTS
+		XML
+		Zip
+)
 
 if(TARGET Poco::Zip)
 	target_link_libraries(mumble
 		PRIVATE
+			Poco::XML
 			Poco::Zip
 	)
 else()