[3.12] gh-119538: Add missing expat build dependencies (GH-119647) (#119686)

xmltok_impl.c and xmltok_ns.c are _included_ in xmltok.c by the C
pre-processor.

(cherry picked from commit 606be66)

Co-authored-by: Erlend E. Aasland <[email protected]>

Author: miss-islington

Makefile.pre.in

@@ -578,7 +578,9 @@ LIBEXPAT_HEADERS= \
 		Modules/expat/utf8tab.h \
 		Modules/expat/xmlrole.h \
 		Modules/expat/xmltok.h \
-		Modules/expat/xmltok_impl.h
+		Modules/expat/xmltok_impl.h \
+		Modules/expat/xmltok_impl.c \
+		Modules/expat/xmltok_ns.c
 
 ##########################################################################
 # hashlib's HACL* library