configure: add stdint.h to x264 and xavs checks

Regression from 4563a86f011b54977b390c72ec3901cace35f8da.
Both need stdint.h included before the respective x264.h and xavs.h.

Old require() used different, separate checks that didn't actually
need stdint.h to work. require2()'s (now require) check_func_headers()
does include stdint.h but only after the custom headers.

For libxavs this would also be consequently fixed by libav's
commit 20abcaa273a6e77d0a2e1a98c643c73562c6f8f2 which wasn't merged yet.
