x86-qemu-fsp: fix wrong multiboot2 header_length in testapp

Author: rizlik

test-app/app_x86_fsp_qemu.c

@@ -50,7 +50,7 @@ __attribute__((aligned(8))) struct multiboot_header mbh = {
     .hdr.magic = 0xe85250d6,
     .hdr.architecture = 0,
     .hdr.checksum = 0,
-    .hdr.header_length = sizeof(struct mb2_header),
+    .hdr.header_length = sizeof(struct multiboot_header),
     .req.type = 1,
     .req.flags = 0,
     .req.size = sizeof(struct mb2_tag_info_req),