util: Disable memstream for Apple builds

Not all SDK versions support open_memstream. Maybe some other day.

Fixes: af8d488ea5 ("util,ac,aco,radv: Cross-platform memstream API")
Reviewed-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/8269>

src/util/memstream.c

@@ -30,7 +30,7 @@
 bool
 u_memstream_open(struct u_memstream *mem, char **bufp, size_t *sizep)
 {
-#ifdef _WIN32
+#if defined(_WIN32)
    bool success = false;
 
    char path[MAX_PATH];
@@ -51,6 +51,8 @@ u_memstream_open(struct u_memstream *mem, char **bufp, size_t *sizep)
    }
 
    return success;
+#elif defined(__APPLE__)
+   return false;
 #else
    FILE *const f = open_memstream(bufp, sizep);
    mem->f = f;