RFR: 8324880: Rename get_stack_trace.h

[Kim Barrett]

Please review this trivial change that renames the file
test/hotspot/jtreg/serviceability/jvmti/thread/GetStackTrace/get_stack_trace.h
to get_stack_trace.hpp, and renames uses of NULL in that file.

The updates to #include lines in using files were performed mechanically.

Testing: mach5 tier1

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Commit messages:
 - rename NULLs in get_stack_trace.hpp
 - rename get_stack_trace.h

Changes: https://git.openjdk.org/jdk/pull/17623/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17623&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324880
  Stats: 15 lines in 9 files changed: 0 ins; 0 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/17623.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17623/head:pull/17623

PR: https://git.openjdk.org/jdk/pull/17623