wireguard: selftests: specify -std=gnu17 for bash

GCC 15 defaults to C23, which bash can't compile under, so specify gnu17
explicitly.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Link: https://patch.msgid.link/20250521212707.1767879-6-Jason@zx2c4.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

Author: zx2c4

tools/testing/selftests/wireguard/qemu/Makefile

@@ -401,6 +401,7 @@ $(BASH_PATH)/.installed: $(BASH_TAR)
 	flock -s $<.lock tar -C $(BUILD_PATH) -xf $<
 	touch $@
 
+$(BASH_PATH)/bash: export CFLAGS_FOR_BUILD += -std=gnu17
 $(BASH_PATH)/bash: | $(BASH_PATH)/.installed $(USERSPACE_DEPS)
 	cd $(BASH_PATH) && ./configure --prefix=/ $(CROSS_COMPILE_FLAG) --without-bash-malloc --disable-debugger --disable-help-builtin --disable-history --disable-progcomp --disable-readline --disable-mem-scramble
 	$(MAKE) -C $(BASH_PATH)