selftest: af_unix: Support compilers without flex-array-member-not-at-end support

Fix:

gcc: error: unrecognized command-line option ‘-Wflex-array-member-not-at-end’

by making the compiler option dependent on its support.

Fixes: 1838731f1072c ("selftest: af_unix: Add -Wall and -Wflex-array-member-not-at-end to CFLAGS.")
Cc: Kuniyuki Iwashima <[e-mail]>
Signed-off-by: Guenter Roeck <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/af_unix/Makefile

···
-CFLAGS += $(KHDR_INCLUDES) -Wall -Wflex-array-member-not-at-end
+top_srcdir := ../../../../..
+include $(top_srcdir)/scripts/Makefile.compiler
+
+cc-option = $(call __cc-option, $(CC),,$(1),$(2))
+
+CFLAGS += $(KHDR_INCLUDES) -Wall $(call cc-option,-Wflex-array-member-not-at-end)
 
 TEST_GEN_PROGS := \
 	diag_uid \