picasa_api_destroy(): do call curl_easy_cleanup()

I see no segfault, not in picasa.c nor in facebook.c, so it must have been fixed already.

Fixes following LeakSanitizer-detected leak:
Direct leak of 36032 byte(s) in 1 object(s) allocated from:
    #0 0x7fe413b94895 in calloc (/usr/lib/x86_64-linux-gnu/libasan.so.1+0x54895)
    #1 0x7fe40f1b5319 (/usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4+0x1d319)
    #2 0x7fe40f1c4a03 in curl_easy_init (/usr/lib/x86_64-linux-gnu/libcurl-gnutls.so.4+0x2ca03)
    #3 0x7fe3ee78850b
    #4 0x7fe3ee78c10e
    #5 0x7fe4137589d3 in dt_imageio_load_modules_storage /home/lebedevri/darktable/src/common/imageio_module.c:306
    #6 0x7fe413758b05 in dt_imageio_init /home/lebedevri/darktable/src/common/imageio_module.c:321
    #7 0x7fe4136fe91f in dt_init /home/lebedevri/darktable/src/common/darktable.c:875
    #8 0x400cb8 in main /home/lebedevri/darktable/src/main.c:24
    #9 0x7fe40bf69b44 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b44)
    #10 0x400bd8 (/usr/local/bin/darktable+0x400bd8)

Author: LebedevRI

src/imageio/storage/picasa.c

@@ -187,8 +187,7 @@ static PicasaContext *picasa_api_init()
 static void picasa_api_destroy(PicasaContext *ctx)
 {
   if(ctx == NULL) return;
-  // FIXME: This is causing a segfault. Probably trying to dereference twice something already freed.
-  // curl_easy_cleanup(ctx->curl_ctx);
+  curl_easy_cleanup(ctx->curl_ctx);
   g_free(ctx->token);
   g_free(ctx->refresh_token);
   g_free(ctx->album_title);