Fix passing multiple XML doc sources to generate_doc_source

[foxandfeature]

CMake functions only received the first XML file due to argument expansion.
Quoting the SOURCES argument ensures multiple XML doc sources are passed correctly.

[enetheru]

My apologies for taking so long to review this simple change, you are quite right.
There is another location that also needs changing and that's in the test cmake file where is uses this function also:
test/CMakeLists.txt

# conditionally add doc data to compile output
if(GODOTCPP_TARGET MATCHES "editor|template_debug")
    file(GLOB_RECURSE DOC_XML LIST_DIRECTORIES NO CONFIGURE_DEPENDS "${CMAKE_CURRENT_SOURCE_DIR}/doc_classes/*.xml")
    target_doc_sources( ${TARGET_NAME} ${DOC_XML} ) # FIXME is missing quotes too.
endif()

I would really appreciate amending this PR to include this fix.