libsodium: fix building with GCC 15.1

libsodium: fix building with GCC 15.1

Force the default C version to -std=gnu17,

to fix the following build error:

<command-line>: error: unknown type name 'thread_local'; did you mean 'pthread_rwlock_t'?
randombytes/internal/randombytes_internal_random.c:132:8: note: in expansion of macro 'TLS'
  132 | static TLS InternalRandom stream = {
      |        ^~~
randombytes/internal/randombytes_internal_random.c:132:27: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'stream'
  132 | static TLS InternalRandom stream = {
      |                           ^~~~~~

[1] https://github.com/openwrt/openwrt/pull/16522#issuecomment-3134756935
[2] https://github.com/openwrt/packages/issues/27122

Tested-by: Georgi Valkov <[email protected]>
Signed-off-by: Lovin Yarn <[email protected]>