libmhash: fix build with gcc 15

GCC 15 switched to C23 by default, which libmhash does not yet support.
So keep using C17.

Signed-off-by: Yi Zhao <yi.zhao@windriver.com>

recipes-security/libmhash/libmhash_0.9.9.9.bb

@@ -25,6 +25,8 @@ SRC_URI[sha256sum] = "56521c52a9033779154432d0ae47ad7198914785265e1f570cee21ab24
 
 inherit autotools-brokensep ptest multilib_header
 
+CFLAGS += "-std=gnu17"
+
 do_install:append() {
     oe_multilib_header mutils/mhash_config.h
 }