[gdb/python] Fix check-include-guards check in python-limited-api-missing.h

The check-include-guards check fails for
gdb/python/python-limited-api-missing.h:
...
check-include-guards....................................................Failed
- hook id: check-include-guards
- exit code: 1

gdb/python/python-limited-api-missing.h:22: wrong symbol in ifndef
...

Fix this by using GDB_PYTHON_PYTHON_LIMITED_API_MISSING_H instead of
GDB_PYTHON_LIMITED_API_MISSING_H.