build: pin -std=c11 for instrument-hooks core.c

The Zig-generated dist/core.c is rejected under C23 (GCC 15's default),
so pin C11 via cflags_c (C-only, so the C++ sources are unaffected).

Author: not-matthias

packages/core/binding.gyp

@@ -8,6 +8,9 @@
       "cflags_cc!": [
         "-fno-exceptions"
       ],
+      "cflags_c": [
+        "-std=c11"
+      ],
       "cflags": [
         "-g",
         "-Wno-maybe-uninitialized",