Force C17 standard in qrexec-lib/validator-test.c

GCC 17 defaults to -std=gnu23, which changes type of u8 string
constants. This breaks the validator, as
qubes_pure_string_safe_for_display expects 'const char *' not 'const
unsigned char *'.

The specific error message is:

validator-test.c:286:48: error: pointer targets in passing argument 1 of ...qubes_pure_string_safe_for_display... differ in signedness [-Werror=pointer-sign]
  286 |     assert(!qubes_pure_string_safe_for_display(u8"\U0001f3f3", 0));
      |                                                ^~~~~~~~~~~~~~
      |                                                |
      |                                                unsigned char *
In file included from validator-test.c:6:
pure.h:199:48: note: expected ...const char *... but argument is of type ...unsigned char *...
  199 | qubes_pure_string_safe_for_display(const char *untrusted_str,
      |                                    ~~~~~~~~~~~~^~~~~~~~~~~~~
validator-test.c:286:48: error: pointer targets in passing argument 1 of ...qubes_pure_string_safe_for_display... differ in signedness [-Werror=pointer-sign]
  286 |     assert(!qubes_pure_string_safe_for_display(u8"\U0001f3f3", 0));
      |                                                ^~~~~~~~~~~~~~
      |                                                |
      |                                                unsigned char *
pure.h:199:48: note: expected ...const char *... but argument is of type ...unsigned char *...
  199 | qubes_pure_string_safe_for_display(const char *untrusted_str,
      |                                    ~~~~~~~~~~~~^~~~~~~~~~~~~

QubesOS/qubes-issues#9807

qrexec-lib/Makefile

@@ -18,7 +18,7 @@ $(pure_objs): CFLAGS += -fvisibility=hidden -DQUBES_PURE_IMPLEMENTATION
 ifeq ($(CHECK_UNREACHABLE),1)
 $(pure_objs): CFLAGS += -DCHECK_UNREACHABLE
 endif
-validator-test: CFLAGS += -UNDEBUG
+validator-test: CFLAGS += -UNDEBUG -std=gnu17
 check: validator-test
 	LD_LIBRARY_PATH=. ./validator-test