boot/syslinux: fix build with gcc-15.x

Fixes build error with gcc-15.x:

drivers/net/3c509-eisa.c:39:26: error:  initialization of 'void (*)(void)'
 from incompatible pointer type 'void (*)(struct nic *, struct eisa_device *)'
 [-Wincompatible-pointer-types]

For a similar fix see https://bugzilla.suse.com/show_bug.cgi?id=1243225

Signed-off-by: Bernd Kuhls <bernd@kuhls.net>
Signed-off-by: Julien Olivain <ju.o@free.fr>

boot/syslinux/syslinux.mk

@@ -65,7 +65,7 @@ define SYSLINUX_BUILD_CMDS
 	$(TARGET_MAKE_ENV) $(MAKE1) \
 		ASCIIDOC_OK=-1 \
 		A2X_XML_OK=-1 \
-		CC="$(TARGET_CC)" \
+		CC="$(TARGET_CC) -std=gnu17" \
 		LD="$(TARGET_LD)" \
 		OBJCOPY="$(TARGET_OBJCOPY)" \
 		AS="$(TARGET_AS)" \