make: Fix 'make syntax-check' failing.

Commit 2a0d75e5 added file python/libvirt-qemu-override.c that contains
code that does not pass "make syntax-check". This patch adds an
exception for this file and the check.

prohibit_always_true_header_tests
python/libvirt-qemu-override.c:17:#undef HAVE_PTHREAD_H
maint.mk: do not test the above HAVE_<header>_H symbol(s);
  with the corresponding gnulib module, they are always true
make: *** [sc_prohibit_always_true_header_tests] Error 1

cfg.mk

@@ -689,7 +689,7 @@ exclude_file_name_regexp--sc_prohibit_VIR_ERR_NO_MEMORY = \
 exclude_file_name_regexp--sc_prohibit_access_xok = ^src/util/util\.c$$
 
 exclude_file_name_regexp--sc_prohibit_always_true_header_tests = \
-  ^python/(libvirt-override|typewrappers)\.c$$
+  ^python/(libvirt-(qemu-)?override|typewrappers)\.c$$
 
 exclude_file_name_regexp--sc_prohibit_asprintf = \
   ^(bootstrap.conf$$|src/util/util\.c$$|examples/domain-events/events-c/event-test\.c$$)