[build] Avoid 'using namespace' in gen-postmortem-metadata.py

This fixes jumbo builds with enabled v8_postmortem_support.

Bug: v8:8838
Change-Id: Idf2efc3b94104ca1921b6de121df1c0aa7f3452c
Reviewed-on: https://chromium-review.googlesource.com/c/1470102
Commit-Queue: Georg Neis <neis@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59549}

tools/gen-postmortem-metadata.py

@@ -317,7 +317,8 @@ header = '''
 #include "src/objects/js-promise.h"
 #include "src/objects/js-regexp-string-iterator.h"
 
-using namespace v8::internal;
+namespace v8 {
+namespace internal {
 
 extern "C" {
 
@@ -333,6 +334,9 @@ STACK_FRAME_TYPE_LIST(FRAME_CONST)
 
 footer = '''
 }
+
+}
+}
 '''
 
 #