[PATCH for-4.21] tools/tests/domid: don't pass -E to sed

[Jan Beulich <jbeulich@suse.com>]

Even the 2018 edition of The Open Group Base Specifications Issue 7 [1]
doesn't name -E as a standard option; only Issue 8 [2] does. As we can
easily avoid anything "extended" about the expression used, drop the -E,
escape the parentheses, and replace the sole + by *.

[1] https://pubs.opengroup.org/onlinepubs/9699919799/
[2] https://pubs.opengroup.org/onlinepubs/9799919799/

Fixes: b3d31533a047 ("tools/tests: introduce unit tests for domain ID allocator")
Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/tools/tests/domid/Makefile
+++ b/tools/tests/domid/Makefile
@@ -10,8 +10,8 @@ include $(XEN_ROOT)/tools/Rules.mk
 TESTS := test-domid
 
 define list-c-headers
-$(shell sed -n -E \
-    's/^[ \t]*# *include[ \t]*[<"]([^">]+)[">].*/\1/p' $(1) 2>/dev/null)
+$(shell sed -n \
+    's/^[ \t]*# *include[ \t]*[<"]\([^">]*\)[">].*/\1/p' $(1) 2>/dev/null)
 endef
 
 # NB: $1 cannot be a list