meson: enable -Wzero-as-null-pointer-constant

Support for C added in gcc 15:
https://github.com/gcc-mirror/gcc/commit/236c0829ee21a179c81b83f0d7f112ca23c47a4d

src/basic/filesystems-gperf.gperf

@@ -1,5 +1,8 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 %{
+#if __GNUC__ >= 15
+_Pragma("GCC diagnostic ignored \"-Wzero-as-null-pointer-constant\"")
+#endif
 #include <linux/magic.h>
 
 #include "filesystems.h"

src/basic/locale-util.c

@@ -276,8 +276,7 @@ int locale_is_installed(const char *name) {
         if (STR_IN_SET(name, "C", "POSIX")) /* These ones are always OK */
                 return true;
 
-        _cleanup_(freelocalep) locale_t loc =
-                newlocale(LC_ALL_MASK, name, 0);
+        _cleanup_(freelocalep) locale_t loc = newlocale(LC_ALL_MASK, name, (locale_t) 0);
         if (loc == (locale_t) 0)
                 return errno == ENOMEM ? -ENOMEM : false;
 

src/basic/lock-util.c

@@ -231,7 +231,7 @@ int lock_generic_with_timeout(int fd, LockType type, int operation, usec_t timeo
                         .sigev_notify = SIGEV_SIGNAL,
                         .sigev_signo = SIGALRM,
                 };
-                timer_t id = 0;
+                timer_t id;
 
                 if (timer_create(CLOCK_MONOTONIC, &sev, &id) < 0) {
                         log_error_errno(errno, "Failed to allocate CLOCK_MONOTONIC timer: %m");