Fix clang-tidy readability-isolate-declaration finding in generated code

[timwoj]

This fixes this finding in a few binpac analyzers:

/Users/tim/Desktop/projects/zeek-master/build/src/analyzer/protocol/gssapi/gssapi_pac.cc:846:9: error: multiple declarations in a single statement reduces readability [readability-isolate-declaration,-warnings-as-errors]
  846 |         const_byteptr t_begin_of_data, t_end_of_data;
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/tim/Desktop/projects/zeek-master/build/src/analyzer/protocol/gssapi/gssapi_pac.cc:862:9: error: multiple declarations in a single statement reduces readability [readability-isolate-declaration,-warnings-as-errors]
  862 |         const_byteptr t_begin_of_data, t_end_of_data;
      |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
8416 warnings generated.

Related to zeek/zeek#4612