Add -DFLAC__NO_DLL to Flac build flags.

This is needed to fix build C2491 errors on Windows:
`definition of dllimport data not allowed` or `definition of dllimport function not allowed`

PiperOrigin-RevId: 667951450

external/flac.BUILD

@@ -49,6 +49,7 @@ flac_copts = [
     "-DNDEBUG",
     "-D_FORTIFY_SOURCE=2",
     "-DFLAC__HAS_OGG=0",
+    "-DFLAC__NO_DLL",  # Static library for Windows
     "-Iexternal/flac/src/libFLAC/include",
     "-Iexternal/flac/include",
 ]