[wasm] Stop requiring TZDIR and TZDEFAULT on WASI (#1248)

Those constants are not used on WASI in Swift side, so there's no
need to require them in the C headers.

Author: kateinoigakukun

Sources/_FoundationCShims/include/_CStdlib.h

@@ -156,7 +156,7 @@
 #ifndef TZDEFAULT
 #define TZDEFAULT    "/etc/localtime"
 #endif /* !defined TZDEFAULT */
-#elif TARGET_OS_WINDOWS
+#elif TARGET_OS_WINDOWS || TARGET_OS_WASI
 /* not required */
 #else
 #error "possibly define TZDIR and TZDEFAULT for this platform"